Cyclopentadienide Ring Research Articles

Hydration of Polycationic [5]Radialene with Quintuple 1,3-Dithiol-2-ylidenes Leads to a New Class of π-Extended Tetrathiafulvalene Scaffold.

[5]Radialene with quintuple 4,5-benzo-1,3-dithiol-2-ylidenes (DTs) easily forms a stable polycationic salt (1)4+ (BF4 - )4 owing to its aromatic character in the central cyclopentadienide ring. In this work, it was found that the polycationic salt (1)4+ underwent a hydration reaction in moist polar solvent to give several unexpected products, namely, an oxygen adduct dicationic salt (2)2+ , a tetrathiafulvalene (TTF) vinylogue (3) with 1,4-dithiine-2(3H)-one moieties, and an oxygen adduct of π-extended TTF with a cyclopentenone core (4). Their molecular structures were fully determined by X-ray crystal-structure analysis. In this reaction, irreversible hydration to the polycationic salt might either initiate the ring expansion and lead to the successive hydration at the specific cationic DT ring (for 3), or promote the transannular reactions to the next DT ring followed by elimination of the DT ring (for 4). Cyclic voltammetry and differential pulse voltammetry measurements for compound 3 indicated the occurrence of multi redox process resulting from electronic delocalization on the vinylogous TTF moiety. The electronic structures of the cationic species of 3 were also investigated by electronic spectra.

N-(acyl)- $$\textit{N}^\prime $$ N ′ -(ferrocenylidene)hydrazines and their nickel(II) complexes: Syntheses, structures and physical properties

The Schiff bases N-(acyl)- $$N^{\prime }$$ -(ferrocenylidene)hydrazines (HFcah (1) and HFcbh (2), where acyl = acetyl in 1 and benzoyl in 2 and H represents the dissociable amide proton) were synthesized in high yields (74 and 81%) by condensation reactions of equimolar amounts of ferrocene-carboxaldehyde and the corresponding acylhydrazines in presence of acetic acid in refluxing methanol. Reactions of $$\text {Ni(OAc)}_{2} \cdot \hbox {4H}_{2}\hbox {O}$$ , the Schiff bases (1 and 2) and $$\hbox {NaOAc}\cdot \hbox {3H}_{2}\hbox {O}$$ in 1:2:4 mole ratio in refluxing methanol afforded the diamagnetic iron(II)-nickel(II)-iron(II) species [ $$\text {Ni(Fcah)}_{2}$$ ] (3) and [ $$\text {Ni(Fcbh)}_{2}$$ ] (4) in 60 and 68% yields, respectively. Both Schiff bases and the two trinuclear complexes were characterized by elemental (CHN) analysis, mass spectrometric, various spectroscopic (IR, UV-Vis and $$^{1}\hbox {H}$$ NMR) and cyclic voltammetric measurements. Molecular structures of 2, 3 and 4 were determined by single crystal X-ray diffraction studies. The cyclopentadienide rings in the ferrocene moieties are essentially in eclipsed conformation in all three structures. Deprotonated Schiff base ligands in each complex ( $$\hbox {Fcah}^{-}$$ in 3 and $$\hbox {Fcbh}^{-}$$ in 4) act as five-membered chelate ring forming azomethine-N and amidate-O donors and assemble a square-planar $${ trans}\hbox {-N}_{2}\hbox {O}_{2}$$ coordination environment around the nickel centre. The redox active 1–4 exhibit an iron centred redox couple in the potential range 0.23–0.50 V (vs. Ag/AgCl). N-(acetyl/benzoyl)- $$N^{\prime }$$ -(ferrocenylidene)hydrazines (HFcah and HFcbh) and their diamagnetic nickel(II) complexes [ $$\hbox {Ni(Fcah)}_{2}$$ ] and [ $$\hbox {Ni(Fcbh)}_{2}$$ ], each containing a bent iron(II)-nickel(II)-iron(II) array, were synthesised and characterised by analytical, spectroscopic and electrochemical measurements. Molecular structures of HFcbh and the two trinuclear complexes have been confirmed by X-ray crystallography.

Unusual metal-ligand charge transfer in ferrocene functionalized μ3-O iron carboxylates observed with Mössbauer spectroscopy

Temperature dependent Mössbauer studies of two ferrocenecarboxylate functionalized {Fe3O} complexes in solid state are reported. It was found that conjugation of ferrocene ring orbitals with the π orbitals of the adjacent carboxylic group promotes a shift of electron density from the ferrocene FeII ion to the cyclopentadienide rings with π-orbital character giving rise to a new type of mixed-valence compound.

Acceleration of Acid-Catalyzed Hydrolysis in a Biphasic System by Sodium Tetracyanocyclopentadienides.

The hydrolysis of tert-butyldimethylsilyl L-menthyl ether (3) in a CH2Cl2-1 M HCl biphasic solvent system was accelerated by the addition of sodium tetracyanocyclopentadienides 1. Particularly, the reaction rate was enhanced using sodium salt 1a-c with a lipophilic substituent on the cyclopentadienide ring. From the results obtained by a triphasic experiment, hydrolysis proceeds via the formation of hydronium ion 2 in the aqueous phase by ion exchange, followed by the transfer of 2 to the CH2Cl2 phase.

Reinvestigation of the aromaticity of mono- and polyphosphacyclopentadienide ions at the DFT level

Aromaticity of mono- and polyphospha-analogues of cyclopentadienide ion has been investigated at the DFT (B3LYP/6-31+G∗∗) level using bond-equalization, hypohomodesmotic reactions, NBO analysis, NICS and 1H NMR chemical shifts criteria. The bond distances are averaged between the corresponding single and double bonds, while hypohomodesmotic reactions give high stabilization energies on CH/P exchange in the cyclopentadienide ring. NBO analysis reveals strong LP→π∗ interactions comparable with those in the parent ion. NICS and 1H NMR chemical shift values support the presence of strong diatropic currents thereby confirming their aromatic character. There is found to be nearly linear correlation between the NICS values and the electron density at the ring critical points.

Modified Carbon Pseudopotential for Use in ONIOM Calculations of Alkyl-Substituted Metallocenes

Nonrelativistic carbon 1s core pseudopotentials are optimized for substituted cyclopentadienide ring carbons for use in integrated molecular orbital molecular orbital (IMOMO) and integrated molecular orbital molecular mechanics (IMOMM) calculations where the Cp ring substituents are not included in the high-level IMOMO or IMOMM subsystem. Use of the optimized pseudopotential within the IMOMO framework leads to significant improvements in predicted carbonyl stretching frequencies for a series of Cp-ring-methylated zirconocenes compared to using a standard carbon pseudopotential. The technology is less successful in the IMOMM implementation.

A Re-Evaluation of the Electrophilic Substitution Reactions of the Ramirez Ylide

Cyclopentadienylidenetriphenylphosphorane (the Ramirez ylide), unexpectedly and contrary to a number of earlier reports, has been shown to be like pyrrole in undergoing electrophilic substitution on the cyclopentadienide ring at either the 3- or the 2-position, depending on the electrophile. Formylation under Vilsmeier conditions and addition of tetracyanoethylene occurs at the 3-position, while activated acetylenes and the nitrosyl electrophile substitute at the 2 position. The 3-formylated product was reduced to the 3-methyl derivative and it also reacted under Knoevenagel conditions to give a number of novel condensation products. The results of single-crystal X-ray crystallographic analyses are given for four of the compounds studied, and a careful 2D NMR analysis of all of the compounds was performed in order to develop a reliable method for the unambiguous assignment of the regiochemistry of adduct formation.

An unprecedented mode of ligation for a bridged amido-cyclopentadienide (constrained geometry) ligand; pi-olefinic interactions with gallium and indium.

The surprising reaction of GaCl3 or InBr3 with the di-Grignard reagent [Me2Si(C5Me4)(N-t-Bu)](MgCl)2 x THF results in salts of the bimetallic anions of composition [X3M[C5Me4(N-t-Bu)]MX2]- (M = Ga, X = Cl; M = In; X = Br) in which the MX2 moiety undergoes an eta2-interaction with one of the double bonds of the localized cyclopentadienide ring.

Coordination of 2,2‘-Bipyridyl and 1,10-Phenanthroline to Substituted Ytterbocenes: An Experimental Investigation of Spin Coupling in Lanthanide Complexes

Addition of 2,2'-bipyridyl to diamagnetic (Me5C5)2Yb(OEt2) gives the brown adduct (Me5C5)2Yb(bipy). The solution 1H NMR and electronic absorption spectra show that the bipyridyl complex is paramagnetic, containing a bipyridyl radical anion, which can also be detected in the solid-state infrared spectrum and by the single-crystal X-ray crystallographic analysis. However, the measured magnetic moment, which varies from less than 1 muB at 5 K to 2.5 muB at 300 K, is higher than expected for (Me5C5)2YbII(bipy0) and less than expected for (Me5C5)2YbIII(bipy-). An electron exchange model for spin coupling between Yb(III), with electron configuration 4f13, and the single unpaired electron in the bipyridyl radical anion is presented, based on comparison with the iodide salt [(Me5C5)2YbIII(bipy0)]+[I]-. Comparing the magnetic susceptibility of (Me5C5)2Yb(phen) with its iodide salt shows similar behavior with phenanthroline as ligand. The extent of paramagnetism and therefore the exchange coupling is changed by the nature of the substituents on the cyclopentadienide rings; electron-withdrawing SiMe3 groups favor Yb(II), while electron-donating alkyl groups stabilize the Yb(III) species. The molecular structures of many of the compounds have been determined in the solid state, and the bond distances and angles are consistent with the interpretation of the magnetism. The ring substituents, and therefore the different magnetic environments about the ytterbium center, also influence the rate of intermolecular exchange of the heterocyclic base ligands in solution; when the ligand is reduced, it exchanges more slowly than in the diamagnetic compounds.

Efficient Photodissociation of Anions from Benzoyl-Functionalized Ferrocene Complexes.

Spectroscopic and photochemical studies of several benzoyl-functionalized ferrocene complexes in nonaqueous solvents are reported. Bands observed above 300 nm in the electronic absorption spectrum of the unsubstituted complex, Fe(eta(5)-C(5)H(5))(2), and assigned to ligand field transitions shift to longer wavelengths and intensify upon introduction of a benzoyl group into one or both cyclopentadienide rings. Such behavior suggests that these transitions have acquired some charge-transfer character. Visible-light (546 nm) irradiation of 1,1'-dibenzoylferrocene, III, dissolved in CH(3)CN, CH(3)OH, or ethyl alpha-cyanopropionate causes ring-metal cleavage to produce the benzoylcyclopentadienide ion, C(6)H(5)C(O)C(5)H(4)(-), and the corresponding half-sandwich cationic complex, Fe[(eta(5)-C(5)H(4))C(O)C(6)H(5)](S)(3)(+) (S is solvent). The disappearance quantum yield, phi(dis), for III is 0.45 in CH(3)OH and 0.28 in ethyl alpha-cyanopropionate and is unaffected by the presence of dissolved O(2), added H(2)O (10 000 ppm), or added methanesulfonic acid (30 ppm). 1,1'-Dibenzoylferrocenes containing substitutents on both phenyl rings undergo photoinduced ring-metal cleavage in CH(3)OH with phi(dis) values very similar to that of III, while monobenzoylferrocenes are appreciably less photoreactive. A mechanism that accommodates the photochemical behavior of benzoyl-functionalized ferrocene complexes is discussed. In addition, a previous suggestion concerning the role of III in the photoinitiated anionic polymerization of an alpha-cyanoacrylate monomer is reconsidered in light of the present study.

Substituted cyclopentadienyl compounds. Part V. NMR and extended Hückel studies of the isomeric preferences of some main group metal cyclopentadienide compounds and a reinvestigation of the crystal structure of thallium(I) tricyanovinylcyclopentadienide

Solution NMR studies have been performed on a series of mono-substituted cyclopentadienide salts, [M +(C 5H 4X −)] (M=Li, Na, K, Cs, Tl; X=CONMe 2, COOMe, COC 6H 5, COMe, CHO, COCO 2C 2H 5, NO 2 and C(CN)C(CN) 2). Population of a fulvenoid isomer in which the dominant bonding mode involves interaction between the cation and the ring substituent heteroatom is enhanced when X is strongly electron accepting and the cation is a hard Lewis acid. An alternate isomer involving a bonding interaction between the cyclopentadienyl carbon atoms and the metal ion predominates when X is weakly accepting and the cation is a soft Lewis acid. A range of compounds between these two isomeric extremes show fluxional behaviour in dimethyl sulphoxide (DMSO) solvent. Within the series of alkali metal salts and for a given X group, the maximum barrier for interconversion of isomers is achieved when M is potassium. This phenomenon is attributed to the balance between Lewis character and solvation of the cation in DMSO. Molecular orbital calculations based on the extended Hückel method were performed to examine the effect of the chemical nature of X and M on the relative energies of the isomers. These suggest that the energy separation is more sensitive to the nature of X than of M, but that the electronegativity of M also plays a role by its effect in shifting the energies into critical π-MO energy ranges. The single crystal X-ray structure of a representative derivative, Tl[C 5H 4C(CN)C(CN) 2] has been reinvestigated. It shows a complex 3D structure in which each thallium is bonded to four nitrogen atoms and a single cyclopentadienide ring, a situation entirely consistent with the behaviour of this compound in solution in which the fulvenoid isomer is dominant.

Poly-linked cyclopentadienide ions: II. Synthesis and properties of the tetracyclopentadienylmethane tetraanion

The title tetraanion ( 1d) has been synthesized via ferrocenyl derivatives. Dynamic 1H NMR spectroscopy indicates the most stable conformation of 1d and the barrier to internal rotation of cyclopentadienide rings.

Electrophilic substitution reaction of meso-ionic sesquifulvalene

The aromatic character of the meso-ionic sesquifulvalene (2) is demonstrated by six electrophilic substitution reactions on the cyclopentadienide ring.

An electron paramagnetic resonance study of tetracyclopentadienyltantalum(IV), [Ta(σ-C5H5)2(η-C5H5)2

X-Band e.s.r. spectra of magnetically dilute solutions of [Ta(σ-C5H5)2(η-C5H5)2] in CHCl3 have been recorded at 77 and at 298 K and are analysed in detail. The spin-Hamiltonian parameters are listed, and are equated to the atomic orbital coefficients in the molecular orbitals involved in bonding. The unpaired electron lies essentially in the 5dx2-y2 metal-ion orbital and it is extensively delocalised on to the cyclopentadienide rings. The σ and π electrons of the cyclopentadienide rings are almost completely delocalised.

An electron paramagnetic resonance study of dichlorobis-(π-cyclopentadienyl)tantalum(IV), (π-C 5H 5) 2TaCl 2

X-band EPR spectra of magnetically-dilute solutions of (π-C 5H 5) 2 TaCl 2 in chloroform have been recorded at 77 K and 298 K and are analysed in detail. The spin-Hamiltonian parameters have been extracted from the spectrum recorded at 298 K, and from spectra of magnetically-dilute glasses taken at 77 K. These parameters, and data obtained from visible—UV spectra, have been used to obtain quantitative descriptions of the bonding in this compound in terms of molecular orbital models. The unpaired electron lies essentially in the 5 d x 2 — y 2 metal-ion orbital and is extensively delocalised onto the chlorine ligands. π-electrons of the cyclopentadienide rings and σ- and π-bonding electrons derived from chlorine ligands are almost completely delocalised.

Electron paramagnetic resonance spectra of the bis-(π-cyclopentadienyl)vanadium(IV) and bis-(π-cyclopentadienyl)niobium(IV) complexes (π-C5H5)2VX2and (π-C5H5)2NbX2

X-Band e.p.r. spectra of magnetically dilute glasses containing (π-C5H5)2VX2 and (π-C5H5)2NbX2(X = Cl, SCN, OCN, and CN) have been recorded at 77 and 290 K and are analysed in detail. The metal-ion spin–orbit coupling constants, ξv and ξNb, for these molecules are estimated to be +133 and +490 cm–1 respectively. Spin Hamiltonian parameters are listed for each substance and are equated to the atomic orbital coefficients in some of the molecular orbitals involved in bonding in these molecules. Except for the cyanides, in each case the unpaired electron lies essentially in a non-bonding ndz2 metal-ion orbital mixed with a small amount of the corresponding metal ion ndx2–y2 orbital, the z-axis coinciding with the C2 axis of the compound; in the cyanides this unpaired electron is delocalised into px orbitals on the ligands. π-Electrons of the cyclopentadienide rings and σ-bonding electrons derived from the other ligands X are almost completely delocalised, π-bonding to the cyclopentadienide residues being stronger than σ-bonding to the ligands X. Several redistribution complexes (π-C5H5)2NbXY have been detected. E.p.r. techniques can be used to distinguish compounds of the type (π-C5H5)2MX2 from other compounds which contain vanadium(IV) or niobium(IV). Hückel LCAO molecular orbital calculations carried out on these molecules appear to give reasonable descriptions of their ground states.

Substitution in the cyclopentadienide anion series: Polymethylation

The reaction of sodium cyclopentadienide with a large amount of methylating agent led to polymethylcyclopentadienes. The resulting product distribution supported our previous conclusions that the cyclopentadienide ring can undergo electrophilic substitution and that a substituent already present on the ring has a directing influence on the electrophile.

Communications - A New Type of Azo Compound by Coupling at the Cyclopentadienide Ring

Communications - A New Type of Azo Compound by Coupling at the Cyclopentadienide Ring