Spectroscopic and thermal characterization of the host–guest interactions between α‐, β‐ and γ‐cyclodextrins and vanadocene dichloride

Host–guest interactions between α‐, β‐ and γ‐cyclodextrins and vanadocene dichloride (Cp₂VCl₂) have been investigated by a combination of thermogravimetric analysis, differential scanning calorimetry, powder X‐ray diffraction and solid‐state and solution electron paramagnetic resonance (EPR) spectroscopy. The solid‐state results demonstrated that only β‐ and γ‐cyclodextrins form 1:1 inclusion complexes, while α‐cyclodextrin does not form an inclusion complex with Cp₂VCl₂. The β‐ and γ‐CD–Cp₂VCl₂ inclusion complexes exhibited anisotropic electron‐⁵¹V (I = 7/2) hyperfine coupling constants whereas the α‐CD–Cp₂VCl₂ system showed only an asymmetric peak with no anisotropic hyperfine constant. On the other hand, solution EPR spectroscopy showed that α‐cyclodextrin (α‐CD) may be involved in weak host–guest interactions in equilibrium with free vanadocene species. Copyright © 2008 John Wiley & Sons, Ltd.     

Investigation of vanadocene(IV) alpha-amino acid complexes: synthesis, structure, and behavior in physiological solutions, human plasma, and blood

This work is focused on investigating the interaction of antitumor active metallocene vanadocene dichloride (Cp2VCl2) and amino acids in aqueous solution at physiological pH. Sixteen vanadocene amino acid complexes [Cp2V(aa)][X] (aa = gly, ala, val, leu, ile, phe, his, and trp; X = Cl, PF6) were prepared and characterized on the basis of spectral measurements (EPR, MS, IR, Raman). Amino acids are coordinated to the vanadocene fragment through the oxygen atom of the carboxylic group and the nitrogen of the amino group, resulting in a five-membered chelate ring. Complexes [Cp2V(val)][PF6] and [Cp2V(ile)][PF6] have been characterized by X-ray structure analyses. It was evidenced that all prepared complexes are stable in both aqueous solutions with physiological pH and in therapeutic NaCl solutions. EPR spectra of vanadocene amino acid complexes in Krebs-Ringer solution in human blood plasma and in whole blood showed that these complexes react with the hydrogen carbonate anion present forming complex Cp2V(O2CO).     

Titanocene(IV) and vanadocene(IV) complexes of dicyanomethanidobenzoate

The new titanocene and vanadocene complexes of the non-linear pseudohalides Cp₂Ti(dcmb)₂, Cp₂VCl(dcmb), (η⁵-C₅H₄Me)₂VCl(dcmb) and Cp₂V(dcmb)₂ were prepared by reaction of titanocene dichloride (Cp₂TiCl₂) and vanadocene dichlorides (Cp₂VCl₂, (η⁵-C₅H₄Me)₂VCl₂) with dicyanomethanidobenzoic acid (dcmbH, PhC(OH)C(CN)₂). These reactions have proven that the dcmb ligand could be coordinated to the central metal by oxygen or nitrogen donor atoms. The bonding mode of the dcmb ligand reflects properties of the central metal. The strongly oxophilic titanium(IV) shows the bonding through oxygen atom while bonding through nitrogen atom was observed for less oxophilic vanadium(IV). The bonding fashion of the dcmb ligands was determined by spectroscopic methods. X-ray diffraction analysis was used for the structure determination of the compounds dcmbH·H₂O, Cp₂Ti(dcmb)₂·CH₂Cl₂, (η⁵-C₅H₄Me)₂VCl(dcmb) and [Cp₂V(OC(Ph)C(CN)C(OMe)NH)][dcmb].     

The interaction of antitumor active vanadocene dichloride with sulfur-containing amino acids

Vanadocene dichloride (1) reacts with sulfur-containing amino acids, cysteine and methionine, giving new complexes with five- or six-membered chelate ring, but the structure of isolated compounds is affected by the pH value of the reaction mixture. Methionine reacts with aqueous 1 in the pH range of 3-8 affording chelate structure [Cp₂V(N,O-met)]Cl (4). Similar reaction with cysteine gives two different products depending on pH. In the acidic solution, the complex [Cp₂V(O,S-cys)]Cl (2) is present, whereas in neutral media the compound [Cp₂V(N,S-cys)] (3) could be identified. On inspection of spectroscopic measurements, particularly EPR and vibrational spectroscopy, it is evident that sulfur atom of amino acid is bonded directly to the vanadium atom of [Cp₂V]²⁺ moiety. For the purpose of comparison the complexes [Cp₂V(O,S-mpa)] (5) and [Cp₂V(N,S-csam)]⁺ (6a) with related chelating ligands, 3-mercaptopropionic acid (mpa) and cysteamine (csam), respectively, were prepared and spectroscopically characterized. The structure of the complex [Cp₂V(N,S-csam)]BPh₄ (6b) was also determined by X-ray diffraction analysis.     

Synthesis and some properties of vanadocene isocyanate and isothiocyanate d1-complexes

Conclusions Vanadocene monochloride and vanadocene dichloride react with organotin and organosilicon pseudohalides, R₃MNCX (R₃M=Bu₃Sn, Me₃Si and X=O, S), to give the corresponding vanadium (IV) d¹-complexes, Cp₂V(NCX)₂ or Cp₂VCl(NCX).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 678–681, March, 1988.     

Synthesis and properties of isocyanate and isothiocyanate d1- and d2-complexes of vanadocene

The reactions of vanadocene and its halides Cp₂VCl and Cp₂VCl₂ with R₃MNCX (M  Sn, Si, X  O, S) and R₂M(NCX)₂ in various molar ratios have been studied. The reactions proceed either by an exchange of groups, with no change in the oxidation state of vanadium, or by an oxidative addition of pseudohalide ligand: V^II → V^III; V^III → V^IV. Oxidative addition results in the formation of (R₃M)₂ or gaseous hydrogen (in the reaction with HCl) in the reaction products.We have prepared the first ever monomeric and readily oxidisable d²-complexes of V^III of Cp₂VNCX-type and asymmetric d¹-complexes of Cp₂V(Cl)NCX type, which, although rather stable in air, undergo disproportionation into symmetric d¹-complexes on heating. In transmetallation reactions the ligand activity is found to increase in the order C1 < NCO < NCS. The complexes were characterised by GLC analysis, IR and ESR spectroscopy. A general scheme for the disproportionation reaction of asymmetric complexes of vanadocene is supported by differential thermal analysis data.     

Synthesis,characterization and structure of the Bis(methyl-cyclopentadienyl)vanadium(IV) carboxylates

The 1,1’-dimethylvanadocene dichloride ((C₅H₄CH₃)₂VCl₂) reacts in aqueous solution with various carboxylic acids giving two different types of complexes. The 1,1’-dimethylvanadocene complexes of monocarboxylic acids (C₅H₄CH₃)₂V(OOCR)₂ (R=H,CCl₃, CF₃, C₆H₅) contain two monodentate carboxylic ligands, whereas oxalic and malonic acids act as chelate compounds of the formula (C₅H₄CH₃)₂V(OOC-A-COO) (A=−, CH₂). The structure of the (C₅H₄CH₃)₂ V(OOCCF₃)₂ complex was determined by single crystal X-ray diffraction analysis. The isotropic and anisotropic EPR spectra of all the complexes prepared were recorded. The obtained EPR parameter values were found to be in agreement with proposed structures.     

An experimental and theoretical study of the Cp2VO2CO: use of 13CO3(2-) for structure investigation of d1-metallocene complexes

EPR study has shown that the anticancer agent vanadocene dichloride (Cp2VCl2) interacts with carbonate contained in physiological solutions. Chelate complex Cp2VO2CO (|A(iso)(51V)| = 175.1 MHz, g(iso) = 1.9861) is the only paramagnetic species formed in the range about the physiological pH (5.5-11.0). The super-hyperfine coupling (|a(iso)(13C)| = 24.1 MHz) was evidenced at measurements using 13C labelled carbonate. The structure of carbonate complex was validated by comparison of observed and theoretical calculated HFC tensors (at the density functional level of theory).     

Vanadocene arsenate complexes: evidence of super‐hyperfine coupling with 75As

The reaction of vanadocene dichloride with sodium arsenate gives Cp₂V(O₂AsO₂H). This compound was identified on the basis of the super‐hyperfine coupling observed in the solution EPR spectrum. The effect of substitution was studied on the ring‐substituted and ansa‐bridged compounds. Copyright © 2010 John Wiley & Sons, Ltd.     

Neodymium and dysprosium diiodides in the synthesis of vanadocene and cobaltocene

The reaction of NdI₂ or DyI₂ with VCl₃ and cyclopentadiene in THF at 65–70 °C without isolation of the intermediates afforded vanadocene in 55 and 68% yields, respectively. An analogous reaction of DyI₂ with CoCl₂ at 50 °C produced Cp₂Co in 38% yield.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2086–2088, October, 2004.     

Vanadocene complexes with asymmetric alkyne ligands, [Cp2V(1-2η-R1C=CR2)]

A number of vanadocene complexes with asymmetrically substituted acetylenes, [Cp₂V-(1-2η-R¹C=CR²)] (R¹ = Ph and SiMe₂Ph; R² = 4-biphenyl, 1-naphthyl, and Ge(CF₃)₃), were obtained and examined by ESR spectroscopy. The isotropic and anisotropic ESR spectra of the complexes containing the phenyl, biphenyl, naphthyl, or dimethyl(phenyl)silyl substituent in the alkyne ligand are typical of d¹ complexes of vanadocene with alkynes. This is not the case of the complex containing the electron-withdrawing tris(trifluoromethyl)germyl group, whose isotropic and anisotropic ESR spectra show anomalous values of g factor and hyperfine coupling constant.     

Thermal decomposition of organo-bielemental vanadium compounds Cp2V(ER3) (ER3 - GeEt3, SnEt3, CH2SiMe3 SeGeEt3)

The thermal decompositon of a number of organo-bielemental vanadium compounds with the general formula Cp₂V(ER₃) (ER₃ - GeEt₃, SnEt₃, CH₂SiMe₃, SeGeEt₃) has been investigated in solids and in solution. The main decomposition products of Cp₂V(SnEt₃) are vanadocene and hexaethyldistannane. Et₃GeH, Et₃GeCp, Cp₂V and CpV(C₅H₄GeEt₃) are formed from Cp₂V (GeET₃) decomposition. Isolated CpV(C₅H₄GeEt₃) is characterized by IR and mass spectra. The decomposition of Cp₂V(CH₂SiMe₃) is accompanied by Me₄Si, Cp₂V and CpV-(C₅H₄CH₂SiMe₃) formation, the latter is identified from the mass spectrum. Triethylgermane, vanadocene, and a diselenide of vanadium are isolated on decomposition of Cp₂V(SeGeEt₃). Based upon the experimental data, mechanisms for the decompositon are proposed.     

Thermal decomposition of dicyclopentadienylarylvanadium compounds

The thermolysis of compounds of the type Cp₂VR (R = aryl) in the solid state has been studied. A distinct increase in thermal stability is observed upon substitution of the ortho-position of the aryl group. Thermal decomposition occurs with formation of RH, Cp₂ V, a vanadocene homologue with the group R substituted in one of the Cp rings and, probably, a vanadocene homologue with two substituted Cp rings. It is shown that the abstraction of the hydrogen atom from the cyclopentadienyl ring, necessary for the formation of RH, is an intermolecular process, whereas the substitution of the aryl group in the Cp ring is intramolecular. A decomposition mechanism is proposed in which the group R is transferred from the vanadium atom to the C₅H₅ ring of the same molecule by interaction with an aryl group of another molecule. The thermal decomposition of Cp₂VR is compared with that of the analogous titanium compounds.     

Ligand-to-ligand charge transfer state in lanthanide complexes containing π-bonded antenna ligands

To design luminescent lanthanide complexes containing both π- and σ-bonded antenna ligands in the coordination sphere, we synthesized 2,2′-bipyridine complexes of Nd, Tb and Gd with tri- and tetraphenyl substituted cyclopentadienyl ligands: [Cp^Ph₃LnCl₂(bipy)(THF)] (Cp^Ph₃ = 1,2,4-triphenylcyclopenta- dienyl, bipy = 2,2′-bipyridine) and [Cp^Ph₄LnCl₂(bipy)(THF)] (Cp^Ph₄ = tetraphenylcyclopentadienyl). Their crystal structures were determined by X-ray diffraction analysis. Optical spectroscopic and crystallographic data indicate the presence of a ligand-to-ligand charge transfer state.     

Reactivity of vanadocene with a nitrile -C identical to N bond activated by a tris(fluorophenyl)borane as Lewis acid: formation of borane adducts of vanada(IV)azirine complexes--EPR evidence for an intramolecular C-F...V interaction

Reaction of vanadocene [V(Cp)2] with "activated" nitrile R1CN.L (L: Lewis acid), obtained by the reaction of borane adducts (L = BR3; R = C6F5, 2,6-F2C6H3, 3,4,5-F3C6H2) with nitriles (CH3CN, F3CC6H4CN), yields the borane adduct of vanada(IV)azirine complexes [V(Cp)2(eta 2-R1C = N.L)]. EPR studies of a fluid solution were conducted on these complexes. A doublet of octets due to the coupling of one unpaired electron of the vanadium with the 51V (I = 7/2) nucleus and to an additional hyperfine coupling to the ortho-F atom borne by the phenyl ring of the borane was elucidated by means of the different Lewis acids used in this work. This EPR behaviour gives evidence for the presence of a C-F...V interaction in a fluid solution with L = B(C6F5)3 and B(2,6-F2C6H3)3. In contrast, the expected eight line EPR pattern is observed with L = B(3,4,5-F3C6H2)3, in which no ortho-F atoms are present in the phenyl ring. A model can be drawn to take into account this flexibility and V...F distances between V and ortho-F atoms are in the expected range for such an interaction.     

Electrochemical studies on organometallic compounds: XI. Electrogeneration of the zirconium(III) and hafnium(III) anions of zirconocene and hafnocene dichlorides

The one-electron reduction of zirconocene and hafnocene dichlorides Cp₂MCl₂ (M = Zr or Hf) yields the corresponding anion Cp₂MCl₂⁻. There is no cleavage of an MCl bond, in contrast to the result in the case of the analogous titanocene and vanadocene dichlorides.     

Oxidation Chemistry of Inorganic Benzene Complexes

The oxidation of the 28 VE cyclo‐E₆ triple‐decker complexes [(Cp^RMo)₂(μ,η⁶:η⁶‐E₆)] (E=P, Cp^R=Cp( 2 a ), Cp*( 2 b ), Cp^Bn( 2 c )=C₅(CH₂Ph)₅; E=As, Cp^R=Cp*( 3 )) by Cu⁺ or Ag⁺ leads to cationic 27 VE complexes that retain their general triple‐decker geometry in the solid state. The obtained products have been characterized by cyclic voltammetry (CV), EPR, Evans NMR, multinuclear NMR spectroscopy, MS, and structural analysis by single‐crystal X‐ray diffraction. The cyclo‐E₆ middle decks of the oxidized complexes are distorted to a quinoid ( 2 a ) or bisallylic ( 2 b , 2 c , 3 ) geometry. DFT calculations of 2 a , 2 b , and 3 persistently result in the bisallylic distortion as the minimum geometry and show that the oxidation leads to a depopulation of the σ‐system of the cyclo‐E₆ ligands in 2 a – 3 . Among the starting complexes, 2 c is reported for the first time including its preparation and full characterization.     

1,1′‐Dimethylvanadocene α‐amino acid complexes: synthesis,characterization and antimicrobial behavior toward Escherichia coli B

Eight water‐soluble 1,1′‐dimethylvanadocene amino acid complexes have been prepared via the reaction of (MeCp)₂VCl₂ ( 2 ) with one equivalent of amino acid (aa) in water affording [(MeCp)₂V( aa )]Cl, where aa is glycine ( 3 ), L ‐alanine ( 4 ), L ‐valine ( 5 ), L ‐leucine ( 6 ), L ‐isoleucine ( 7 ), L ‐phenylalanine ( 8 ), L ‐histidine ( 9 ) and L ‐tryptophane ( 10 ). All prepared complexes have been characterized by EPR, IR and Raman spectroscopy, elemental analysis and mass spectrometry. Molecular structures of [(MeCp)₂V(ala)]BPh₄·CH₃OH ( 11 ), [(MeCp)₂V(leu)]PF₆ ( 12 ) and [(MeCp)₂V(ile)]PF₆ ( 13 ) were determined by X‐ray diffraction analysis. Cytotoxic properties of complexes 2–10 were investigated toward Escherichia coli B and compared with analogical unsubstituted vanadocene compounds ( 1, 14–21 ). The results showed that 1,1′‐dimethylvanadocene amino acid complexes have identical or slightly higher antiproliferative activity then their unsubstituted analogs. Copyright © 2006 John Wiley & Sons, Ltd.     

Preparation and spectral studies of dithiophosphate and dithiophosphinate complexes of dicyclopentadienylvanadium(IV)

The dithiophosphate and dithiophosphinate complexes

[S₂-PR₂]⁻ (R = Et, OEt, OPrⁱ) can be conveniently prepared by reaction of Cp₂V with the disulphides [SP(S)R₂]₂ in THF; all products have been characterized by IR and EPR spectroscopy and conductivity measurements. The EPR spectra of the complexes have been recorded in solution at room temperature and in glasses; they show isotropic interaction of the unpaired electron with the ⁵¹V and ³¹P nuclei, which can be accounted for in terms of delocalization through the sulphur atoms.     

Structural chemistry of titanium and aluminium bimetallic hydride complexes VI. Molecular structure and physico-chemical properties of (η5-C5 H5) 2Ti(μ2-H)2 Al(Cl) (μ2 -H)2Ti(η5-C5H 5)2, a catalyst-precursor for hydrogenation in the [(η5-C5H5)2TiCl]2-LiAIH4 system

The complex (Cp₂Ti)₂AlH₄Cl has been isolated from the catalytic system (Cp₂TiCl)₂-LiAlH₄, which is a precursor of the catalyst for the hydrogenation and isomerization of olefins. This complex has been studied by X-ray diffraction. The complex forms rhomboidal crystals with unit cell dimensions a = 10.414, b = 11.998, c = 16.008 Å, space group P2₁2₁2₁, Z = 4, and density ϱ_calc = 1.40 g/cm³. The Cp₂Ti moieties are linked to the Al atom via double hydrogen bridges; the Cl atom is bonded to the Al atom. Analysis of the EPR spectral data and some chemical properties of (Cp₂Ti)₂AlH₄Cl solutions has led us to suggest a mechanism for the formation of the catalytically active species upon interaction of this compound with olefins and solvating solvents.