Preparation of Organometallic Ruthenium–Arene–Diaminotriazine Complexes as Binding Agents to DNA

The reactions of two diaminotriazine ligands 2,4‐diamino‐6‐(2‐pyridyl)‐1,3,5‐triazine (2‐pydaT) and 6‐phenyl‐2,4‐diamino‐1,3,5‐triazine (PhdaT) with ruthenium–arene precursors led to a new family of ruthenium(II) compounds that were spectroscopically characterized. Four of the complexes were cationic, with the general formula [(η⁶‐arene)Ru(κ²‐N,N‐2‐pydaT)Cl]X (X=BF₄, TsO; arene=p‐cymene: 1.BF4 , 1.TsO arene=benzene: 2.BF4 , 2.TsO ). The neutral cyclometalated complex [(η⁶‐p‐cymene)Ru(κ²‐C,N‐PhdaT*)Cl] ( 3 ) was also isolated. The structures of complexes 2.BF4 and 3.H2O were determined by X‐ray diffraction. Complex 1.BF4 underwent a partial reversible‐aquation process in water. UV/Vis and NMR spectroscopic measurements showed that the reaction was hindered by the addition of NaCl and was pH‐controlled in acidic solution. At pH 7.0 (sodium cacodylate) Ru–Cl complex 1.BF4 was the only species present in solution, even at low ionic strength. However, in alkaline medium (KOH), complex 1.BF4 underwent basic hydrolysis to afford a Ru–OH complex ( 5 ). Fluorimetric studies revealed that the interaction of complex 1.BF4 with DNA was not straightforward; instead, its main features were closely linked to ionic strength and to the [DNA]/complex ratio. The bifunctional complex 1.BF4 was capable of interacting concurrently through both its p‐cymene and 2‐pydaT groups. Cytotoxicity and genotoxicity studies showed that, contrary to the expected behavior, the complex species was biologically inactive; the formation of a Ru–OH complex could be responsible for such behavior.     

Dioxomolybdenum(VI) complexes containing 1-alkyl-2-ethyl-3-hydroxy-4-pyridin-4(1H)-ones

Dioxomolybdenum(VI) complexes derived from ethyl maltol (2-ethyl-3-hydroxy-4-pyrone) and 1-alkyl-2-ethyl-3-hydroxy-4-pyridin-4(1H)-ones have been prepared and characterized using physical methods including ¹H- and ¹³C-n.m.r. spectroscopy, i.r., elemental analysis, and X-ray diffraction for the pyrone and 3-methylpyridine pyridinone derivatives. The octahedral complexes have the general formula cis-MoO₂L₂ where L = the deprotonated pyrone or pyridinone ligand.     

Retracted: Acid‐Catalyzed Aquation of Ni(II)‐Hydrazone Complexes: Kinetics and Solvent Effect

Complexes of the type [Ni(L)(H₂O)]Cl₂·nH₂O, where L = 2‐pyridyl‐3‐isatinbishydrazone ligands, have been synthesized and characterized on the bases of elemental analysis, molar conductance, IR, electronic spectra, and thermal analysis (TGA and DTA). Acid‐catalyzed aquation of the Ni(II) isatin‐bishydrazone complexes was followed spectrophotometrically in various water–methanol and water–acetone mixtures at temperature 298 K. Kinetic behavior of the acid aquation is a linear rate law, indicating that the acid‐catalyzed aquation of these complexes in water–methanol and water–acetone mixtures follows a rate law with k_obs = k₂[H⁺]. The effect of the mole fraction of the ganic solvent, i.e., methanol and acetone, on the acid aquation has been analyzed; the decrease in the rate constant values with increasing of the methanol or acetone ratios is attributable to the effect of the co‐organic solvent on the initial states of the acid aquation by the destabilization of the H⁺ ion.     

Acceptorless Dehydrogenative Oxidation of Secondary Alcohols Catalysed by Cp*IrIII–NHC Complexes

A series of new Ir^III complexes with carbene ligands that contain a range of benzyl wingtip groups have been prepared and fully characterised by NMR spectroscopy, HRMS, elemental analysis and X‐ray diffraction. All the complexes were active in the acceptorless dehydrogenation of alcohol substrates in 2,2,2‐trifluoroethanol to give the corresponding carbonyl compounds. The most active complex bore an electron‐rich carbene ligand; this complex was used to catalyse the highly efficient and chemoselective dehydrogenation of a wide range of secondary alcohols to their respective ketones, with turnover numbers up to 1660. Mechanistic studies suggested that the turnover of the dehydrogenation reaction is limited by the H₂‐formation step.     

Am(m)ines Make the Difference: Organoruthenium Am(m)ine Complexes and Their Chemistry in Anticancer Drug Development

With the aim of systematically studying fundamental structure–activity relationships as a basis for the development of Ru^II arene complexes (arene=p‐cymene or biphenyl) bearing mono‐, bi‐, or tridentate am(m)ine ligands as anticancer agents, a series of ammine, ethylenediamine, and diethylenetriamine complexes were prepared by different synthetic routes. Especially the synthesis of mono‐, di‐, and triammine complexes was found to be highly dependent on the reaction conditions, such as stoichiometry, temperature, and time. Hydrolysis and protein‐binding studies were performed to determine the reactivity of the compounds, and only those containing chlorido ligands undergo aquation or form protein adducts. These properties correlate well with in vitro tumor‐inhibiting potency of the compounds. The complexes were found to be active in anticancer assays when meeting the following criteria: stability in aqueous solution and low rates of hydrolysis and binding to proteins. Therefore, the complexes least reactive to proteins were found to be the most cytotoxic in cancer cells. In general, complexes with biphenyl as arene ligand inhibited the growth of tumor cells more effectively than the cymene analogues, consistent with the increase in lipophilicity. This study highlights the importance of finding a proper balance between reactivity and stability in the development of organometallic anticancer agents.     

Petrorhagiosides A – D,New γ‐Pyrone Derivatives from Petrorhagia saxifraga Link

Four novel γ‐pyrone (=4H‐pyran‐4‐one) metabolites, petrorhagiosides A–D, along with four known analogs, have been isolated from the MeOH extract of Petrorhagia saxifraga, a perennial herbaceous plant typical of Mediterranean vegetation. The structures of the new compounds were established on the basis of extensive spectroscopic analyses including 1D‐ an 2D‐NMR (¹H,¹H‐DQ‐COSY, TOCSY, HSQC, CIGAR‐HMBC, and HSQC‐TOCSY) experiments.     

Hydrogen Bonding in Phosphine Oxide/Phosphate–Phenol Complexes

To develop a new solvent‐impregnated resin (SIR) system for the removal of phenols and thiophenols from water, complex formation by hydrogen bonding of phosphine oxides and phosphates is studied using isothermal titration calorimetry (ITC) and quantum chemical modeling. Six different computational methods are used: B3LYP, M06‐2X, MP2, spin component‐scaled (SCS) MP2 [all four with 6‐311+G(d,p) basis set], a complete basis set extrapolation at the MP2 level (MP2/CBS), and the composite CBS‐Q model. This reveals a range of binding enthalpies (ΔH) for phenol–phosphine oxide and phenol–phosphate complexes and their thio analogues. Both structural (bond lengths/angles) and electronic elements (charges, bond orders) are studied. Furthermore, solvent effects are investigated theoretically by the PCM solvent model and experimentally via ITC. From our calculations, a trialkylphosphine oxide is found to be the most promising extractant for phenol in SIRs, yielding ΔH=?14.5 and ?9.8 kcal mol^?1 with phenol and thiophenol, respectively (MP2/CBS), without dimer formation that would hamper the phenol complexation. In ITC measurements, the ΔH of this complex was most negative in the noncoordinating solvent cyclohexane, and slightly less so in π–π interacting solvents such as benzene. The strongest binding is found for the dimethyl phosphate–phenol complex [?15.1 kcal mol^?1 (MP2/CBS)], due to the formation of two H‐bonds (P?O???H‐O‐ and P‐O‐H???O‐H); however, dimer formation of these phosphates competes with complexation of phenol, and would thus hamper their use in industrial extractions. CBS‐Q calculations display erroneous trends for sulfur compounds, and are found to be unsuitable. Computationally relatively cheap SCS‐MP2 and M06‐2X calculations did accurately agree with the much more elaborate MP2/CBS method, with an average deviation of less than 1 kcal mol^?1.     

Hydrolysis mechanism of anticancer Pd(II) complexes with coumarin derivatives: a theoretical investigation

The hydrolysis reaction mechanisms of a new generation Pd(II) anticancer drugs containing coumarin derivatives have been investigated combining density functional theory with the conductor-like dielectric continuum model approach. The first and the second aquation processes have been explored for the cis and trans counterpart of title PdL₂ complex. Two possibilities for the second hydrolysis process have been analyzed for both compounds. From our data emerge that cis and trans PdL₂ compounds have a different behavior in water. cis-isomer readily undergo first hydrolysis process generating a mono-aqua complex while both the pathways investigated for the second aquation reaction are more energetically demanding, suggesting that the mono-aqua complex could act as active species. On the contrary, for trans-compound, both the investigated reactions for the second aquation process occur by overcoming activation barriers comparable with that found for the first hydrolysis reaction. According to our data, trans-oriented PdL₂ drug could undergo degradation process generating non-active compounds with the consequent lack of pharmacological activity.     

Biological Effect,Aquation, and Kinetics of Oxidation of Bis(2‐aminobenzothiazole)dichlorocobalt(II) Complex by Periodate in Aqueous Acidic Medium

The biological effect, aquation, and kinetics of oxidation of bis(2‐aminobenzothiazole)dichlorocobalt(II) complex by periodate in aqueous acidic solutions were studied. The complex exhibited a broad resistance toward the studied pathogens. The average value of the aquation constant was calculated spectrophotometrically as 2.55 × 10^?5 mol² dm^?6. Kinetics of the oxidation reaction showed first‐order dependence on each reactant concentration and increased by increasing pH over the 3.80–4.80 range, 35–50°C, and decreased by increasing the ionic strength over the 0.1–0.5 mol dm^?3 range. The polymerization of acrylonitrile was taken as an evidence for an inner‐sphere mechanism through the formation of free radical intermediates of Co(III) complexes, which were slowly converted to the final Co(III)products.     

Chiral Supramolecular Switches Based on (R)‐Binaphthalene–Bipyridinium Guests and Cucurbituril Hosts

We designed and synthesized the three molecular tweezers 1 a – c ⁴⁺ containing an electron acceptor 4,4′‐bipyridinium (BPY²⁺) unit in each of the two arms and an (R)‐2,2′‐dioxy‐1,1′‐binaphthyl (BIN) unit that plays the role of chiral centre and the hinge of the structure. Each BPY²⁺ unit is connected to the BIN hinge by an alkyl chain formed by two‐ ( 1 a ⁴⁺), four‐ ( 1 b ⁴⁺), or six‐CH₂ ( 1 c ⁴⁺) groups. The behavior of 1 a – c ⁴⁺ upon chemical or photochemical reduction in the absence and in the presence of cucurbit[8]uril (CB[8]) or cucurbit[7]uril (CB[7]) as macrocyclic hosts for the bipyridinium units has been studied in aqueous solution. A detailed analysis of the UV/Vis absorption and circular dichroism (CD) spectra shows that the helicity of the BIN unit can be reversibly modulated by reduction of the BPY²⁺ units, or by association with cucurbiturils. Upon reduction of 1 a – c ⁴⁺ compounds, the formed BPY^{+ .} units undergo intramolecular dimerization with a concomitant change in the BIN dihedral angle, which depends on the length of the alkyl spacers. The alkyl linkers also play an important role in association to cucurbiturils. Compound 1 a ⁴⁺, because of its short carbon chain, associates to the bulky CB[8] in a 1:1 ratio, whereas in the case of the smaller host compound CB[7] a 1:2 complex is obtained. Compounds 1 b ⁴⁺ and 1 c ⁴⁺, which have longer linkers, associate to two cucurbiturils regardless of their sizes. In all cases, association with CB[8] causes an increase of the BIN dihedral angle, whereas the formation of CB[7] complexes causes an angle decrease. Reduction of the CB[8] complexes results in an enhancement of the BPY^{+ .} dimerization with respect to free 1 a – c ⁴⁺ and causes a noticeable decrease of the BIN dihedral angle, because the BPY^{+ .} units of the two arms have to enter into the same macrocycle. The dimer formation in the CB[8] complexes characterized by a 1:2 ratio implies the release of one macrocycle showing that the binding stoichiometry of these host–guest complexes can be switched from 1:2 to 1:1 by changing the redox state of the guest. When the reduction is performed on the CB[7] complexes, dimer formation is totally inhibited, as expected because the CB[7] cavity cannot host two BPY^{+ .} units.     

Solution cation‐binding properties of segmented polyethylene oxide–A 13C NMR spectroscopic study

Two segmented polyethylene oxides, SPEO‐3 and SPEO‐4, were prepared using a novel transetherification methodology. Their structures were confirmed by ¹H and ¹³C NMR spectroscopy. The complexation of these SPEO's with alkali–metal ions in solution was investigated by ¹³C NMR spectroscopy. The mole‐fraction method was used to determine the complexation ratio of SPEO with LiClO₄ at 25 °C, which showed that these formed 1:1 (polymer repeat unit/salt) complexes. The association constant, K, for the complex formation was calculated from the variation of the chemical shift values with salt concentration, using a standard nonlinear least‐square fitting procedure. The maximum change in chemical shift (Δδ) and the K values suggest that both SPEO‐3 and SPEO‐4 formed stronger complexes with lithium salts than with sodium salts. Unexpectedly, the K values were found to be different, when the variation of δ of different carbons was used in the fitting procedure. This suggests that several possible complexed species may be in equilibrium with the uncomplexed one. Structurally similar model compounds were also prepared and their complexation studies indicated that all of them also formed 1:1 complexes with Li salts. Interestingly, it was observed that the polymers gave higher K values suggesting the formation of more stable complexes in polymers when compared to the model analogues. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2635–2644, 2000     

Ruthenium–Arene–β‐Carboline Complexes as Potent Inhibitors of Cyclin‐Dependent Kinase 1: Synthesis,Characterization and Anticancer Mechanism Studies

A series of Ru^II–arene complexes ( 1 – 6 ) of the general formula [(η⁶‐arene)Ru(L)Cl]PF₆ (arene=benzene or p‐cymene; L=bidentate β‐carboline derivative, an indole alkaloid with potential cyclin‐dependent kinases (CDKs) inhibitory activities) is reported. All the complexes were fully characterized by classical analytical methods, and three were characterized by X‐ray crystallography. Hydrolytic studies show that β‐carboline ligands play a vital role in their aqueous behaviour. These complexes are highly active in vitro, with the most active complex 6 displaying a 3‐ to 12‐fold higher anticancer activity than cisplatin against several cancer cell lines. Interestingly, the complexes are able to overcome cross‐resistance to cisplatin, and show much lower cytotoxicity against normal cells. Complexes 1 – 6 may directly target CDK1, because they can block cells in the G2M phase, down‐regulate the expression of CDK1 and cyclin B1, and inhibit CDK1/cyclin B in vitro. Further mechanism studies show that the complexes can effectively induce apoptosis through mitochondrial‐related pathways and intracellular reactive oxygen species (ROS) elevation.     

Factors Affecting DNA–DNA Interstrand Cross‐Links in the Antiparallel 3′–3′ Sense: A Comparison with the 5′–5′ Directional Isomer

Reported herein is a study of the unusual 3′–3′ 1,4‐GG interstrand cross‐link (IXL) formation in duplex DNA by a series of polynuclear platinum anticancer complexes. To examine the effect of possible preassociation through charge and hydrogen‐bonding effects the closely related compounds [{trans‐PtCl(NH₃)₂}₂(μ‐trans‐Pt(NH₃)₂{NH₂(CH₂)₆NH₂}₂)]⁴⁺ (BBR3464, 1 ), [{trans‐PtCl(NH₃)₂}₂(μ‐NH₂(CH₂)₆NH₂)]²⁺ (BBR3005, 2 ), [{trans‐PtCl(NH₃)₂}₂(μ‐H₂N(CH₂)₃NH₂(CH₂)₄)]³⁺ (BBR3571, 3 ) and [{trans‐PtCl(NH₃)₂}₂{μ‐H₂N(CH₂)₃‐N(COCF₃)(CH₂)₄}]²⁺ (BBR3571‐COCF₃, 4 ) were studied. Two different molecular biology approaches were used to investigate the effect of DNA template upon IXL formation in synthetic 20‐base‐pair duplexes. In the “hybridisation directed” method the monofunctionally adducted top strands were hybridised with their complementary 5′‐end labelled strands; after 24 h the efficiency of interstrand cross‐linking in the 5′–5′ direction was slightly higher than in the 3′–3′ direction. The second method involved “postsynthetic modification” of the intact duplex; significantly less cross‐linking was observed, but again a slight preference for the 5′–5′ duplex was present. 2D [¹H, ¹⁵N] HSQC NMR spectroscopy studies of the reaction of [¹⁵N]‐ 1 with the sequence 5′‐d{TATACATGTATA}₂ allowed direct comparison of the stepwise formation of the 3′–3′ IXL with the previously studied 5′–5′ IXL on the analogous sequence 5′‐d(ATATGTACATAT)₂. Whereas the preassociation and aquation steps were similar, differences were evident at the monofunctional binding step. The reaction did not yield a single distinct 3′–3′ 1,4‐GG IXL, but numerous cross‐linked adducts formed. Similar results were found for the reaction with the dinuclear [¹⁵N]‐ 2 . Molecular dynamics simulations for the 3′–3′ IXLs formed by both 1 and 2 showed a highly distorted structure with evident fraying of the end base pairs and considerable widening of the minor groove.     

C−H Bond Activation/Arylation Catalyzed by Arene–Ruthenium–Aniline Complexes in Water

Water‐soluble arene–ruthenium complexes coordinated with readily available aniline‐based ligands were successfully employed as highly active catalysts in the C?H bond activation and arylation of 2‐phenylpyridine with aryl halides in water. A variety of (hetero)aryl halides were also used for the ortho‐C?H bond arylation of 2‐phenylpyridine to afford the corresponding ortho‐ monoarylated products as major products in moderate to good yields. Our investigations, including time‐scaled NMR spectroscopy and mass spectrometry studies, evidenced that the coordinating aniline‐based ligands, having varying electronic and steric properties, had a significant influence on the catalytic activity of the resulting arene–ruthenium–aniline‐based complexes. Moreover, mass spectrometry identification of the cycloruthenated species, {(η⁶‐arene)Ru(κ²‐C,N‐phenylpyridine)}⁺, and several ligand‐coordinated cycloruthenated species, such as [(η⁶‐arene)Ru(4‐methylaniline)(κ²‐C,N‐phenylpyridine)]⁺, found during the reaction of 2‐phenylpyridine with the arene–ruthenium–aniline complexes further authenticated the crucial roles of these species in the observed highly active and tuned catalyst. At last, the structures of a few of the active catalysts were also confirmed by single‐crystal X‐ray diffraction studies.     

Inclusion compounds of cucurbit[<Emphasis Type="Italic">n</Emphasis>]urils with metal complexes

A new class of supramolecular compounds—inclusion compounds of metal complexes encapsulated in organic macrocyclic cavitands cucurbit[n]urils (CB[n], C_6n H_6n N_4n O_2n , n = 7−10)—has been surveyed. A unique combination of a rather rigid hydrophobic intramolecular cavity and negatively charged portals favors the formation of stable host-guest compounds. Basic methods of synthesis of inclusion compounds of CB[n] with metal complexes have been reported, and the structures of the resulting products isolated as crystals and characterized by X-ray crystallography have been considered. The effect of encapsulation on the geometric and spectral characteristics of the complexes and their redox properties has been traced. It has been shown that encapsulation in CB[n] can lead to a change in the reactivity of the complexes in thermolysis and isomerization and aquation reactions. Encapsulation of biologically active metal complexes in CB[n] is a promising strategy for designing new-generation prolonged-action pharmaceuticals.     

Kinetics of interaction and complexation of the palladium(II) ion with new para-substituted-phenylazo-R-acid chromophores

The kinetics of complex formation between the Pd^II ion and the new phenylazo-R-acid chromophore [HL] and it is para-substituted derivatives {X = H, Cl, CO₂H, and SO₂NH₂}, [p-X-HL], have been studied at various temperatures. The observed rate constant is given by:k_obs=/k₁ K₁/K[H⁺]+k₂ K₂}[p-X-HL]A mechanism involving initial formation of a 1:1 complex between the Pd^II ion and [p-X-HL], followed by aquation as the rate-determining step, is proposed. The activation enthalpy and entropy for the aquation were determined. The rate of formation is greatly effected by electron-withdrawing substitutents in the para position of the phenylazo-R-acid ligand. The solid complex of p-SO₂NH₂-phenylazo-R-acid has been prepared and characterized by elemental analysis, and FT-IR spectra. The p-substituted-phenylazo-R-acid ligands are very active against C. albicans fungi, so that these compounds can be used for the chemotherapy of candidiasis and other fungal skin diseases.     

Two Rare α‐Pyrone (=2H‐Pyran‐2‐one) Derivatives from Gentiana rhodantha Franchet

Two new rare α‐pyrone (=2H‐pyran‐2‐one) derivatives, rhodanthpyrones A and B ( 1 and 2 , resp.), together with fourteen known compounds, 3 – 16 , were isolated from the whole plants of Gentiana rhodantha. The structures of these compounds were elucidated by spectroscopic analyses. This is the first report on the occurrence of α‐pyrone derivatives in the genus Gentiana.     

Asymmetric Donor–π‐Acceptor‐Type Benzo‐Fused Aza‐BODIPYs: Facile Synthesis and Colorimetric Properties

Novel aza‐diisoindolylmethene and their BF₂‐chelating complexes (benzo‐fused aza‐BODIPYs) were synthesized on a large scale and in a facile manner from phthalonitrile in tBuOK‐DMF solution. The unique asymmetric donor–π‐acceptor structure facilitates B? N bond detachment in the presence of trifluoroacetic acid (TFA) in dichloromethane, resulting in sharp color change from red to colorless, with over 250 nm hypsochromic shift in the absorption maximum. This colorimetric process can be reversed by adding a very small amount of proton‐accepting solvents or compounds. A ¹H and ¹¹B NMR spectroscopy study and also density functional theory (DFT) calculations suggest that TFA‐induced B? N bond cleavage may disrupt the whole π‐conjugation of the BODIPY molecule, resulting in significant colorimetric behavior.     

Application of flavonoids – quercetin and rutin – as new matrices for matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometric analysis of Pt(II) and Pd(II) complexes

Attempts are being made to overcome the resistance of tumour cells to platinum (Pt) drugs by the synthesis of new generations of Pt complexes, and it is important to find appropriate and simple methods for the characterization of those novel complexes. The additional applicability of such a method for the analysis of the interactions of metal complexes with biomolecules would be advantageous. Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS) seems to possess the capability to become this method of choice, since it could be applied to low‐mass complexes as well as for the analysis of large biomolecules. In this work the applicability of flavonoids – quercetin and rutin – as matrices for MALDI‐TOFMS analysis of dichlorido(ethylendiamine)platinum(II) ([PtCl₂(en)]), dichlorido(diaminocyclohexane)platinum(II) ([PtCl₂(dach)]) and chloride (diethylenetriamine) palladium(II) chloride ([PdCl(dien)]Cl) complexes is demonstrated. Spectra of Pt(II) and Pd(II) complexes recorded in the presence of quercetin and rutin are rather simple: Pt(II) complexes generate [M+Na]⁺ or [M+K]⁺ions, whereas the investigated Pd(II) complex gives ions generated by the loss of one Cl^? or HCl. Flavonoids give a relatively small number of well‐defined ions in the low‐mass region (at m/z 303.3 for quercetin and m/z 633.5 for rutin). Quercetin and rutin can be applied in much lower concentrations than other common MALDI matrices and require rather low laser intensity. We speculate that flavonoids stabilize the structures of the metal complexes and that they may be useful for the analysis of other biologically active metal complexes, thus implying their broader applicability. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis,structure characterization and catalytic activity of lanthanide complexes containing an ansa carbonous‐bridged cyclopentadienyl—thienyl ligand

A kind of new lanthanocene complex with an ansa carbonous‐bridged cyclopentadienyl/aromatic heterocycle ligand was prepared and characterized. Based on the data of elemental analyses, MS and IR, they were presumed to be solvent‐free complexes (cyclo‐C₄H₃SCMe₂C₅H₄)₂LnCl [Ln = Er (1), Dd ( 2 ), Y ( 3 ), Sm ( 4 )]. These complexes were effective for the polymerization of methyl methacrylate in the presence of co‐catalyst. When AlEt₃ and NaH (nanometric) were used as different co‐catalysts, the lanthanocene complexes 1–4 showed different catalytic behavior. These differences resulted from the formation of different active species. The catalyst system (cyclo‐C₄H₃SCMe₂C₅H₄)₂LnCl/NaH (nanometric) showed high catalytic activity (yield ≥ 95% and Mη > 10⁵) in a short time at the ambient temperature. Copyright © 2008 John Wiley & Sons, Ltd.