Metallocene interactions with DNA and DNA-processing enzymes.
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Antineoplastic Agents↗
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Electronic structures of mixed-sandwich complexes of cyclopentadienyl and Hydrotris(pyrazolyl)borate ligands with 3d transition metals.
The electronic and magnetic properties of a series of mixed-sandwich complexes MCp(R)Tp (Cp(R) = Cp or Cp; Tp = hydrotris(pyrazolyl)borate; M = V, Cr, Fe, Co or Ni) have been studied and compared to their homoleptic analogues, MCp(R)(2) and MTp(2). Solid-state magnetic susceptibility measurements and EPR spectroscopic data indicate that complexes with d(3), d(6), and d(8) configurations are similar electronically to their metallocene analogues, except for FeCpTp, which displays a spin equilibrium (S = 0 if S = 2) in solution which was investigated by variable- temperature NMR spectroscopy. The d(2) complex [VCpTp](+) displays magnetic behavior consistent with an orbitally nondegenerate ground state. The d(4) species CrCpTp has a high-spin (S = 2) ground state. The d(7) species CoCpTp is high spin (S = 3/2) whereas its Cp analogue and [NiCpTp](+) are both low-spin (S = 1/2) species. The optical spectra of the d(3), d(6), and d(8) complexes were assigned in a fashion similar to the analogous metallocenes and ligand-field parameters (delta(1) = delta-sigma gap, delta(2) = sigma-pi gap for d-orbitals in axial symmetry) calculated. The analysis shows that for 15-electron species the total ligand-field splitting, delta(TOT), is larger than for their metallocene analogues, whereas for the 18-electron case Delta(TOT) is smaller and for 20-electron systems delta(TOT) is approximately the same. In all cases delta(2) is substantially reduced compared to the metallocenes, and in the majority of cases delta(1) is markedly larger. DFT calculations were performed to investigate further the nature of the ligand environment on the frontier orbitals in these complexes. Orbital energies and compositions were calculated and compared for a series of homoleptic and mixed-sandwich complexes of Ni(II) and across the 1st transition series for MCp(R)Tp species. The ability of Tp (vs Cp) to act as a delta-donor (with respect to the principal molecular axis) imparts significant ligand antibonding character to the delta-orbitals and results in decreased epsilon(pi)-epsilon(delta) values compared to the metallocenes and an increased tendency toward high-spin complexes in the mixed-sandwich complexes. Structure calculations were performed for CrCpTp, [VCpTp](+), and CoCpTp which show substantial distortions from ideal axial symmetry in their crystal structures. The origins of these distortions were confirmed to arise from unequal occupancy of near-degenerate delta- and pi-levels.
Journal Article↗
Organometallic anticancer agents: cellular uptake and cytotoxicity studies on thiol derivatives of the antitumor agent molybdocene dichloride.
The effect of aqueous solubility, charge, and lability of four thiol derivatives of the antitumor metallocene molybdocene dichloride (Cp(2)MoCl(2)) on the cell uptake and cytotoxicity against V79 Chinese hamster lung cells has been determined. Addition of 4-thiol-2,3,5,6-tetrafluorobenzoic acid, 1-thio-beta-d-glucose, and 1-thio-2,3,4,5-tetraacetyl-beta-d-glucose to aqueous solutions of molybdocene dichloride afforded the corresponding metallocenes in which the deprotonated thiols are coordinated to the metal center. These metallocenes were studied, along with the previously reported glutathione derivative Cp(2)Mo(GS)(2), which has been proposed to be formed from molybdocene dichloride in blood plasma. In contrast to Cp(2)MoCl(2) which rapidly loses the chloride ligands to form a positively charged aquated complex at pH 7, the thiol derivatives are stable to ligand hydrolysis in 50 mM salt at 37 degrees C for 24 h. Cytotoxicity values determined by a modified 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay gave an IC(50) value of 350 microM for molybdocene dichloride with V79 cells, with similar values obtained with human breast MCF-7 (620 microM) and ovarian 2008 (700 microM) cell lines. The water-soluble thiol derivatives were not cytotoxic, while the acetylated sugar derivative was insoluble in water or aqueous dimethyl sulfoxide. Cell uptake experiments in which the molybdenum content in cells treated with each metallocene for 24 h was measured by graphite furnace atomic absorption spectroscopy showed that the fluorinated aromatic derivative was most efficiently transported into cells, followed by molybdocene dichloride, with the lowest uptake observed for Cp(2)Mo(GS)(2) and the glucose derivative. The cell uptake results do not correlate with overall charge of the complexes or the measured IC(50) values. The distinct cytotoxicity and cell uptake profiles of Cp(2)MoCl(2) compared with Cp(2)Mo(GS)(2) show that while rapid coordination of Cp(2)MoCl(2) to glutathione occurs in water at pH 7, significant deactivation of molybdocene dichloride by conversion to Cp(2)Mo(GS)(2) does not occur in cells.
Animals↗
Characterization of the low-energy electronic excited states of benzoyl-substituted ruthenocenes.
Electronic absorption and resonance Raman spectral studies of benzoylruthenocene (BRc) and 1,1'-dibenzoylruthenocene (DRc) indicate that the low-energy electronic excited states of these 4d(6) metallocenes possess metal-to-ligand charge transfer (MLCT) character. While this MLCT contribution should weaken the metal-ring bonding in the excited state, neither compound is photosensitive in nonhalogenated solvents such as methanol, acetonitrile, and cyclohexane. In contrast, irradiating BRc and DRc in the good electron-accepting solvent, carbon tetrachloride, results in ring loss via a pathway that appears to originate from a charge-transfer-to-solvent excited state. Both metallocenes function as photoinitiators for the anionic polymerization of ethyl 2-cyanoacrylate, and the kinetics and mechanism of this process have been investigated. Comparing the present results on BRc and DRc with those reported earlier for the corresponding benzoyl-substituted ferrocene compounds reveals some interesting commonalities and differences between the excited-state properties of these 3d and 4d metallocenes.
Algorithms↗
Alpha-amino acids with metallocenyl side chains.
A straightforward method for the synthesis of enantiomerically pure bis(valine)metallocenes is presented. Derivatives of lithium cyclopentadienylvaline 1a, b were obtained by addition of the (R)- or (S)-Schöllkopf reagents to 6,6-dimethylfulvene as single enantiomers and gave with FeCl2 or [RuCl2(dmso)4] the chiral metallocenes [Fe[C5H4-CMe2-[C4H2N2(OMe)2iPr]]2] (2a, b) and [Ru[C5H4-CMe2-[C4H2N2(OMe)2iPr]]2] (3a, b). Complex 2b was hydrolyzed to the ferrocenylene-bis(valine-methylester) [[Fe[C5H4-CMe2-CH(NH3+)COOMe]2]2+(Cl-)2] (7) without racemization. Complex 7 could be used as ligand and was treated with [[Cp*IrCl2]2] to afford [Fe[C5H4-CMe2-CH(COOMe)(NH2-IrCp*Cl2)]2] (10). The reactions of 1 with CoCl2, [Re(CO)5Br], [[(cod)RhCl2]2] (cod= 1,5-cyclooctadiene) or [Cp*MCl3] (M= Ti, Zr) gave the cyclopentadienyl complexes [[Co[C5H4-CMe2-[C4H2N2(OMe)2iPr]]2]+ I-] (11) and [Re[C5H4-CMe2-[C4H2N2(OMe)2iPr]](CO)3] (13), [(C8H12)Rh[C5H4-CMe2-[C4H2N2(OMe)2(iPr)]]] (14). [[Rh[C5H4-CMe2-[C4H2N2(OMe)2(iPr)]]I]2(mu-I)2] (15), [Cp*Cl2Ti-[C5H4-CMe2-[C4H2N2(OMe)2(iPr)]]] (16), and [Cp*Cl2Zr[C5H4-CMe2-[C4H2N2(OMe)2(iPr)]]] (17), with chiral valine derivatives as substituents on the cyclopentadienyl ring and with excellent diastereoselectivities. Also the Seebach reagent (Boc-BMI) or O'Donnell reagent could be added to 6,6-dimethylfulvene to give the lithium cyclopentadienides Li[C5H4-CMe2-[C3H2(tBu)(N-Boc)(NMe)O]] (18) and Li[C5H4-CMe2-CH(NCPh2)(COOEt)] (21), which formed the ferrocene derivatives [Fe[C5H4-CMe2-[C3H2(tBu)(N-Boc)(NMe)O]]2] (19) and [Fe[C5H4-CMe2-CH(NCPh2)(COOEt)]2] (22). The stable cobaltocinium cation in 11 and the complex 19 could be hydrolyzed to the metallocenes 12 and [Fe(C5H4-CMe2-CH(NH3+)(COO-)]2] (20) with two valines in the 1,1'-position. The structures of 2a, b, 11, 15, and 16 were determined by X-ray diffraction and confirm the diastereomeric purity of the compounds.
Amino Acids↗
Homogeneous single-component betaine Ziegler-Natta catalysts derived from (butadiene)zirconocene precursors.
(Butadiene)zirconocene adds B(C(6)F(5))(3) at a terminal diene carbon atom to yield the zirconocene-(mu-hydrocarbyl)-borate betaine Cp(2)Zr[C(4)H(6)-B(C(6)F(5))(3)] (4). The dipolar complex 4 contains a distorted pi-allyl moiety and features an additional stabilizing Zr-F-C(arene) coordination. Under kinetic control, an isomeric betaine system is formed, characterized by an internal Zr(+).CH(2)[B](-) ion-pair interaction, that rearranges to 4 upon heating. A great variety of ansa-metallocene(butadiene) complexes and related systems cleanly form analogous metallocene-(mu-conjugated diene)-borate betaines upon treatment with B(C(6)F(5))(3) and related Lewis acids. Most of these systems represent very active homogeneous single-component Ziegler-Natta catalysts for alpha-olefin polymerization and copolymerization. In addition, these betaine catalysts are ideally suited for carrying out mechanistic studies in active Ziegler-Natta catalyst systems. They allow for an experimental observation of the first alkene insertion step at the active single-component catalyst. This feature has been used for studying the mechanism of transfer of the stereochemical information from the bent metallocene backbone and for an experimental characterization of the energy profile of the alkene addition/alkene insertion reaction sequence in active homogeneous Ziegler-Natta systems. The neutral dipolar single-component catalysts (e.g., 4) produce a polyolefin-modified R'-(CHR-CH(2))-C(4)H(6)-B(C(6)F(5))(3)(-) counterion at the end of the initiation period upon entering into the repetitive active catalytic cycle.
Betaine↗
Zirconocene-catalyzed propene polymerization: a quenched-flow kinetic study.
The kinetics of propene polymerization catalyzed by ansa-metallocenes were studied using quenched-flow techniques. Two catalyst systems were investigated, (SBI)ZrMe2/Al(i)Bu3/[Ph3C][CN[B(C6F5)3]2] (1:100:1) at 25.0 degrees C and (SBI)ZrCl(2)/methylalumoxane at 40.0 degrees C (Al:Zr = 2400:1) (SBI = rac-Me(2)Si(1-Indenyl)2). The aims of the study were to address fundamental mechanistic aspects of metallocene-catalyzed alkene polymerizations, catalyst initiation, the quantitative correlation between catalyst structure and the rate of chain propagation, and the nature of dormant states. One of the most important but largely unknown factors in metallocene catalysis is the distribution of the catalyst between dormant states and species actively involved in polymer chain growth. Measurements of polymer yield Y versus reaction time t for propene concentrations [M] = 0.15-0.59 mol L(-1) and zirconocene concentrations in the range [Zr] = (2.38-9.52) x 10(-5) mol L(-1) for the borate system showed first-order dependence on [M] and [Zr]. Up to t approximately 1 s, the half-life of catalyst initiation is comparable to the half-life of chain growth; that is, this phase is governed by non-steady-state kinetics. We propose a rate law which takes account of this and accurately describes the initial rates. Curve fitting of Y(t) data provides an apparent chain growth rate constant k(p)(app) on the order of 10(3) L mol(-1) s(-1). By contrast, the evolution with time of the number-average polymer molecular weight, which is independent of the concentration of catalyst involved, leads to a k(p) which is an order of magnitude larger, (17.2 +/- 1.4) x 10(3) L mol(-1) s(-1). The ratio k(p)(app)/k(p) = 0.08 indicates that under the given conditions only about 8% of the total catalyst is actively engaged in chain growth at any one time. The system (SBI)ZrCl(2)/methylalumoxane is significantly less active, k(p)(app) = 48.4 +/- 2.7 and k(p) = (6 +/- 2) x 10(2) L mol(-1) s(-1), while, surprisingly, the mole fraction of active species is essentially identical, 8%. Evidently, the energetics of the chain growth sequence are strongly modulated by the nature of the counteranion. Increasing the counteranion/zirconium ratio from 1:1 to 20:1 has no influence on catalyst activity. These findings are consistent with a model of closely associated ion pairs throughout the chain growth sequence. For the borate system, propagation is approximately 6000 times faster than initiation, while for the MAO catalyst, k(p)/k(i) approximately 800. Polymers obtained at 25 degrees C show 0.1-0.2 mol % 2,1-regioerrors, and end-group analysis identifies 2,1-misinsertions as the main cause for chain termination (66%), as compared to 34% for the vinylidene end groups. The results suggest that 2,1-regioerrors are a major contributor to the formation of dormant species, even at short reaction times.
Journal Article↗
Lanthanide(III)/actinide(III) differentiation in coordination of azine molecules to tris(cyclopentadienyl) complexes of cerium and uranium.
Reaction of azine molecules L with the trivalent metallocenes [M(C5H4R)3](M = Ce, U; R = But, SiMe3) in toluene gave the Lewis base adducts [M(C5H4R)3(L)](L = pyridine, 3-picoline, 3,5-lutidine, 3-chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine), except in the cases of M = U and L = 3-chloropyridine, pyridazine, pyrazine and s-triazine where oxidation of U(III) was found to occur. In the pairs of analogous compounds of Ce(III) and U(III), i.e.[M(C5H4But)3(L)](L = pyridine, picoline) and [M(C5H4SiMe3)3(L)](L = pyridine, lutidine, pyrimidine and dimethylpyrazine), the M-N and average M-C distances are longer for M = Ce than for M = U; however, within a series of azine adducts of the same metallocene, no significant variation is noted in the M-N and average M-C distances. The equilibria between [M(C5H4R)3], L and [M(C5H4R)3(L)] were studied by 1H NMR spectroscopy. The stability constants of the uranium complexes, KUL, are greater than those of the cerium counterparts, KCeL. The values of KML are much greater for R = SiMe3 than for R = But and a linear correlation is found between the logarithms of KML and the hydrogen-bond basicity pKHB scale of the azines. Thermodynamic parameters indicate that the enthalpy-entropy compensation effect holds for these complexation reactions. Competition reactions of [Ce(C5H4R)3] and [U(C5H4R)3] with L show that the selectivity of L in favour of U(III) increases with the [small pi] donor character of the metallocene and is proportional to the pi accepting ability of the azine molecule, measured by its reduction potential.
Actinoid Series Elements↗
Diffusion of limonene in polyethylene.
Diffusion coefficients of limonene in various linear low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) resins have been determined from sorption data using a thermogravimetric methodology. From these data, one can determine whether polymer synthesis parameters such as the choice of catalytic process or co-monomer result in substantial differences in how much food packaging additives might migrate to food. For example, LLDPE is currently manufactured using either one of two distinct catalytic processes: Ziegler-Natta (ZN) and metallocene, a single-site catalyst. ZN catalysis is a heterogeneous process that has dominated polyolefin synthesis over the last half-century. It involves a transition metal compound containing a metal-carbon bond that can handle repeated insertion of olefin units. In contrast, metallocene catalysis has fewer than 20 years of history, but has generated much interest due to its ability to produce highly stereospecific polymers at a very high yield. In addition to high stereospecificity, metallocene-catalysed polymers are significantly lower in polydispersity than traditional ZN counterparts. Absorption and desorption testing of heat-pressed films made from LLDPE and LDPE resins of varying processing parameters indicates that diffusion coefficients of limonene in these resins do not change substantially.
Absorption↗
Helically chiral ferrocene peptides containing 1'-aminoferrocene-1-carboxylic acid subunits as turn inducers.
We present a detailed structural study of peptide derivatives of 1'-aminoferrocene-1-carboxylic acid (ferrocene amino acid, Fca), one of the simplest organometallic amino acids. Fca was incorporated into di- to pentapeptides with D- and L-alanine residues attached to either the carboxy or amino group, or to both. Crystallographic and spectroscopic studies (circular dicroism (CD), IR, and NMR) of about two dozen compounds were used to gain a detailed insight into their structures in the solid state as well as in solution. Four derivatives were characterized by single-crystal X-ray analysis, namely Boc-Fca-Ala-OMe (16), Boc-Fca-D-Ala-OMe (17), Boc-Fca-beta-Ala-OMe (18), and Boc-Ala-Fca-Ala-Ala-OMe (21) (Boc=tert-butyloxycarbamyl). CD spectroscopy is an extremely useful tool to elucidate the helical chirality of the metallocene core. Unlike in all other known ferrocene peptides, the helical chirality of the ferrocene is governed solely by the chirality of the amino acid attached to the N terminus of Fca. Depending on the degree of substitution of both cyclopentadiene (Cp) rings, different hydrogen-bonding patterns are realized. (1)H NMR and IR spectroscopy, together with the results from X-ray crystallography, give detailed information regarding not only the hydrogen-bonding patterns of the compounds, but also the equilibria between different conformers in solution. Differences in chemical shifts of NH protons in dimethyl sulfoxide ([D(6)]DMSO) and CDCl(3), that is, the variation ratio (vr), is used for the first time as a measure of the hydrogen-bonding strength of individual COHN bonds in ferrocenoyl peptides. In dipeptides with one intramolecular hydrogen bond between the pendant chains, for example, in dipeptide 16, an equilibrium between hydrogen-bonded and open forms is observed, as testified by a vr value of around 0.5. Higher peptides, such as tetrapeptide 21, are able to form two intramolecular hydrogen bonds stabilizing one single conformation in CDCl(3) solution (vr approximately 0). Due to the low barrier of Cp-ring rotation, new and unnatural hydrogen-bonding patterns are emerging. The systematic work described herein lays a solid foundation for the rational design of metallocene peptides with unusual structures and properties.
Carboxylic Acids↗
[Enzymatic hydroxylation of ruthenocene and osmocene by liver microsomes].
In buffer solution (pH 7,4) ruthenocene and osmocene are far more stable than ferrocene. The metallocenes ruthenocene and osmocene are metabolized by microsomes of mouse liver in the presence of NADPH and O2. The Km-values are similar for both metallocenes, the rate of metabolism is higher for osmocene that for ruthenocene.
Animals↗
Homoleptic lanthanide amides as homogeneous catalysts for alkyne hydroamination and the Tishchenko reaction.
The homoleptic bis(trimethylsilyl)amides of Group 3 metals and lanthanides of the general type [Ln[N(SiMe3)2]3] (1) (Ln=Y, lanthanide) represent a new class of Tishchenko precatalysts and, to a limited extent, precatalysts for the hydroamination/cyclization of aminoalkynes. It is shown that 1 is the most active catalyst for the Tishchenko reaction. This contribution presents investigations on the scope of the reaction, substrate selectivity, lanthanide-ion size-effect, and kinetic/mechanistic aspects of the Tishchenko reaction catalyzed by 1. The turnover frequency is increased by the use of large-center metals and electron-withdrawing substrates. The reaction rate is second order with respect to the substrate. While donor atoms, such as nitrogen, oxygen, or sulfur, on the substrate decrease the turnover frequency, 1 shows a tolerance for a large number of functional groups. For the hydroamination/cyclization of aminoalkynes, 1 is less active than the well-known metallocene catalysts. On the other hand, 1 is much more readily accessible (one-step synthesis or commercially available), than the metallocenes and might therefore be an attractive alternative catalyst.
Journal Article↗
Relevance of the electronegativity of boron in eta5-coordinating ligands: regioselective monoalkylation and monoarylation in cobaltabisdicarbollide [3,3'-Co(1,2-C2B9H11)2]- clusters.
Regioselective monoalkylation and monoarylation in cobaltabisdicarbollide clusters has been achieved starting from Cs[8-I-3,3'-Co(1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))] by cross-coupling reactions between a B-I fragment and an appropriate Grignard reagent in the presence of a Pd catalyst and CuI. A considerable number of monoalkylated and monoarylated derivatives have been synthesized, which allowed study of the influence of boron in metallocene-type ligands and the effect of alkyl and aryl substituents on boron in boron anionic clusters. Experimental data from UV/Vis spectroscopy, E(1/2) measurements, and X-ray diffraction analysis, and supported by EHMO and ab initio analyses, indicate that the participation of metal d orbitals in the HOMO is less than that in typical metallocene complexes. This can be explained in terms of the lower electronegativity of boron compared to carbon. Related to this is the -I character of alkyl groups when bonded to boron in boron anionic clusters, contrary to the common belief that alkyl groups are generally electron-releasing moieties.
Journal Article↗
Molecular genotoxicity profiles of apoptosis-inducing vanadocene complexes.
Metallocene complexes containing vanadium induce apoptosis in human cancer cells by an as yet unknown mechanism and may therefore be useful as a new class of cytotoxic anticancer drugs. Ultrastructural studies showing the formation of metallocene-DNA complexes prompted the hypothesis that their mechanism of action may resemble the DNA damage induced by cisplatin. Molecular genotoxicity testing provides insights into the mechanisms of action of new chemotherapeutic agents. Therefore, we determined the effects of three cytotoxic vanadocene complexes, vanadocene dichloride, vanadocene dithiocyanate, and vanadocene dioxycyanate, on genomic stability using the yeast DEL recombination assay and transcriptional activation of genotoxic stress-specific promoters in human HepG2 cells using the CAT-Tox(L) assay. Cisplatin caused an 11-fold increase of recombination frequency in yeast and induced transcriptional activation of the DNA damage-associated promoters such as the minimum promoter containing p53 response elements and the GADD45 promoter in addition to activating the promoters for c-fos, heat shock protein 70, metallothionine IIa, and the minimum promoter containing nuclear factor kappa(kappa)B response elements. In contrast to cisplatin, vanadocene complexes did not increase the DEL recombination frequency in yeast nor did they activate any of the DNA damage-associated promoters in HepG2 cells. Vanadocene complexes triggered activation of the c-fos promoter without affecting the minimum promoter containing p53 response elements or the GADD45 promoter. These results indicate that the apoptotic signal of vanadocene complexes is not triggered by primary DNA damage and it does not require p53 induction, thereby disproving the hypothesis that it mechanistically resembles the cytotoxic action of cisplatin.
Antineoplastic Agents↗
Do penta- and decaphospha analogues of lithocene anion and beryllocene exist? Analysis of stability, structure, and bonding by hybrid density functional study.
Stability in penta- and decaphospha analogues of lithocene anion and beryllocene is investigated by complete structural optimization at the B3LYP/6-31G level. Natural bond orbital analysis is carried out to examine the bonding between the metal and the ligands. The heterolytic dissociation energies of 667 and 608 kcal/mol predicted by B3LYP/6-311+G//B3LYP/6-31G calculations for CpBeP(5) and (P(5))(2)Be are comparable with the observed value of 635 +/- 15 kcal/mol in ferrocene. The high stability in CpBeP(5) and (P(5))(2)Be shows that these species are isolable under appropriate conditions. Lithocene anion and its phospha analogues possess lower stability toward dissociation into ionic fragments. A novel observation of the present study is that CpBeP(5) and (P(5))(2)Be have lowest energies when the two planar ligands are arranged perpendicular to each other such that one of the ligands, cyclo-P(5), is eta(1)-coordinated while the second ligand is eta(5)-coordinated to Be. The resulting structure having C(s)() point group (denoted as C(s)()(p)) is predicted to be 22 and 28 kcal/mol lower than the staggered sandwich geometry in CpBeP(5) and (P(5))(2)Be, respectively, at the B3LYP/6-311+G//B3LYP/6-31G level. In the analogous lithocene anions [CpLiP(5)](-) and [(P(5))(2)Li](-) also the C(s)()(p) structures are found to be the lowest energy structures, though their relative stabilities are small. We also characterized the geometry with both ligands eta(1)-coordinated to the metal in a linear arrangement having the D(2)(h)() point group in the decaphospha analogues [(P(5))(2)Li](-) and (P(5))(2)Be. This structure is found to be higher in energy than the C(s)()(p) structure. The D(2)(h)() structure could not be located as a potential minimum in the biscyclopentadienyl complexes and their pentaphospha analogues. Both the C(s)()(p) and D(2)(h)() structures are characterized for the first time in metallocenes. The D(2)(h)() structure seems to be a unique feature in the decaphospha metallocenes under consideration. Covalent bond formation between beryllium and phosphorus atom P(1) of eta(1)-(cyclo-P(5)) is more pronounced (bond orders 0.43-0.49) than that between Be and C(1) of eta(1)-Cp (bond orders 0.24-0.27). Though both eta(1)-coordinated cyclo-P(5) and Cp exhibit C(2)(v)() point groups, bond alternation is less pronounced in the former. The Wiberg P-P bond orders in the eta(1)-(cyclo-P(5)) of CpBeP(5) and (P(5))(2)Be having C(s)()(p) structures are in the range 1.29-1.47. These ring bond orders indicate that the P(5) ring retains aromaticity to a large extent in the eta(1)-mode of bonding with Be. Second-order perturbational energy analysis of the Fock matrix in the natural bond orbital basis reveals that there is a significant stabilizing interaction of approximately 123 kcal/mol between the lone pair orbital of P(1) and the 2s orbital of Be in the C(s)()(p) structures.
Journal Article↗
Synthesis and Reactivity of Organosamarium Diarylpnictide Complexes: Cleavage Reactions of Group 15 E-E and E-C Bonds by Samarium(II).
(C(5)Me(5))(2)Sm (2 equiv) reacts with Ph(2)EEPh(2) to give (C(5)Me(5))(2)SmEPh(2) (E: P, 1; As, 2), while (C(5)Me(5))(2)Sm(THF)(2) (2 equiv) reacts with Ph(2)EEPh(2) to give (C(5)Me(5))(2)Sm(EPh(2))(THF) (E: P, 3; As, 4). 3 and 4 are also available from the reactions of 1 and 2 with THF. 3 and 4 undergo further reaction to produce the THF ring-opened products (C(5)Me(5))(2)Sm[O(CH(2))(4)EPh(2)](THF) (E: P, 5; As, 6).(C(5)Me(5))(2)Sm (4 equiv) reacts with Ph(2)EEPh(2) to give the mixed-valent (C(5)Me(5))(2)Sm(&mgr;-EPh(2))Sm(C(5)Me(5))(2) (E: P, 7; As, 8). These compounds are also available from the reaction of 1 and 2 with (C(5)Me(5))(2)Sm. The X-ray crystal structure of 2, crystallized from hexanes (P2(1)/n; a = 26.188(24) Å, b = 9.911(10) Å, c = 23.280(23) Å, beta = 97.150(12) degrees, V = 5995(2) Å(3), D(calcd) = 1.488 Mg/m(3); Z = 8; T = 156 K), revealed, in addition to a conventional seven-coordinate bent metallocene geometry with 2.698 Å Sm-C(C(5)Me(5)) and 2.970 Å Sm-As average distances, two very different Sm-As-C(Ph) angles, 74.2 and 118.7 degrees. As a result, one phenyl group is closer to the metal (2.901 Å minimum Sm-C distance). 4, crystallized from toluene (P2(1)/n; a = 10.713(9) Å, b = 14.143(11) Å, c = 21.620(16) Å, beta = 101.08(6) degrees, V = 3215(4) Å(3), D(calcd) = 1.492 Mg/m(3); Z = 4; T = 163 K), and 6, crystallized from hexanes (P2(1)/n; a = 9.3958(16) Å, b = 22.245(3) Å, c = 17.931(3) Å, beta = 96.497(11) degrees, V = 3724(1) Å(3), D(calcd) = 1.416 Mg/m(3); Z = 4; T = 163 K), have conventional eight-coordinate, bent metallocene structures.
Journal Article↗
Synthesis and Structure of Bis(pi-cyclopentadienyl)vanadium(IV) 1,10-Phenanthroline and 2,2'-Bipyridine Compounds and Their Interactions with Artificial Membranes.
The reaction of in situ generated Cp(2)V(OTf)(2) (Cp = cyclopentadienyl; OTf = O(3)SCF(3)) with excess 1,10-phenanthroline and 2,2'-bipyridine yields the d(1) vanadocene coordination compounds [Cp(2)V(phen)][OTf](2) (1) and [Cp(2)V(bpy)][OTf](2) (2), respectively. The compounds have been characterized by UV-vis and EPR spectroscopy and by cyclic voltammetry. The complexes have relatively low vanadium(IV)-vanadium(III) reduction potentials (-0.62 V vs Cp(2)Fe(+/0) in acetonitrile). Structures of 1 and 2 have been determined by X-ray crystallography. Compound 1 crystallized in a monoclinic system, space group P2(1)/n, with a = 10.2763(5) Å, b = 18.1646(9) Å, c = 13.5741(7) Å, beta = 99.4150(10) degrees, and Z = 4. Refinement of its structure by full-matrix least-squares techniques gave final residuals R = 0.040 and R(w) = 0.096. Compound 2 crystallized in a monoclinic system, space group P2(1)/c, with a = 10.6451(6) Å, b = 18.3863(10) Å, c = 12.6993(7) Å, beta = 98.6220(10) degrees, and Z = 4. Refinement of its structure by full-matrix least-squares techniques gave final residuals R = 0.046 and R(w) = 0.101. The two nitrogen atoms and centroids of the two cyclopentadienyl rings for both compounds occupy a distorted tetrahedral geometry around the vanadium(IV) center. The chelated ring plane is inclined closer to one of the neighboring Cp rings with the tilt more evident in 1 ( approximately 8 degrees ) than 2 ( approximately 4 degrees ). The membrane interactions of these compounds and the titanium analogues, [Cp(2)Ti(phen)][OTf](2) (3) and [Cp(2)Ti(bpy)][OTf](2) (4), have been studied with zwitterionic unilamellar liposomes as artificial membranes. We show that the ability of metallocenes to enhance the permeability of a liposomal membrane depends on the hydrophobicity, as well as the size and planarity of the ancillary chelated ligands, but not the nature of the central metal ion. Also provided is evidence that metallocene-induced permeability changes in artificial membranes are not caused by lipid peroxidation.
Journal Article↗
Kinetics of initiation, propagation, and termination for the [rac-(C(2)H(4)(1-indenyl)(2))ZrMe][MeB(C(6)F(5))(3)]-catalyzed polymerization of 1-hexene.
Metallocene-catalyzed polymerization of 1-alkenes offers fine control of critical polymer attributes such as molecular weight, polydispersity, tacticity, and comonomer incorporation. Enormous effort has been expended on the synthesis and discovery of new catalysts and activators, but elementary aspects of the catalytic processes remain unclear. For example, it is unclear how the catalyst is distributed among active and dormant sites and how this distribution influences the order in monomer for the propagation rates, for which widely varying values are reported. Similarly, although empirical relationships between average molecular weights and monomer have been established for many systems, the underlying mechanisms of chain termination are unclear. Another area of intense interest concerns the role of ion-pairing in controlling the activity and termination mechanisms of metallocene-catalyzed polymerizations. Herein we report the application of quenched-flow kinetics, active site counting, polymer microstructure analysis, and molecular weight distribution analysis to the determination of fundamental rate laws for initiation, propagation, and termination for the polymerization of 1-hexene in toluene solution as catalyzed by the contact ion-pair, [rac-(C(2)H(4)(1-indenyl)(2))ZrMe][MeB(C(6)F(5))(3)] (1) over the temperature range of -10 to 50 degrees C. Highly isotactic (>99% mmmm) poly-1-hexene is produced with no apparent enchained regioerrors. Initiation and propagation processes are first order in the concentrations of 1-hexene and 1 but independent of excess borane or the addition of the contact ion-pair [PhNMe(3)][MeB(C(6)F(5))(3)]. Active site counting and the reaction kinetics provide no evidence of catalyst accumulation in dormant or inactive sites. Initiation is slower than propagation by a factor of 70. The principal termination process is the formation of unsaturates of two types: vinylidene end groups that arise from termination after a 1,2 insertion and vinylene end groups that follow 2,1 insertions. The rate law for the former termination process is independent of the 1-hexene concentration, whereas the latter is first order. Analysis of (13)C-labeled polymer provides support for a mechanism of vinylene end group formation that is not chain transfer to monomer. Deterministic modeling of the molecular weight distributions using the fundamental rate laws and kinetic constants demonstrates the robustness of the kinetic analysis. Comparisons of insertion frequencies with estimated limits on the rates of ion-pair symmetrization obtained by NMR suggest that ion-pair separation prior to insertion is not required, but the analysis requires assumptions that cannot be validated.
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