New tripodal N-heterocyclic carbene chelators for small molecule activation

In an effort to develop new tripodal N-heterocyclic carbene (NHC) ligands for small molecule activation, two new classes of tripodal NHC ligands TIME^R and TIMEN^R have been synthesized. The carbon-anchored tris(carbene) ligand system TIME^R (R = Me, t-Bu) forms bi- or polynuclear metal complexes. While the methyl derivative exclusively forms trinuclear 3:2 complexes [(TIME^Me)₂M₃]³⁺ with group 11 metal ions, the tert-butyl derivative yields a dinuclear 2:2 complex [(TIME^t-Bu)₂Cu₂]²⁺ with copper(I). The latter complex shows both “normal” and “abnormal” carbene binding modes and accordingly, is best formulated as a bis(carbene)alkenyl complex. The nitrogen-anchored tris(carbene) ligands TIMEN^R (R = alkyl, aryl) bind to a variety of first-row transition metal ions in 1:1 stoichiometry, affording monomeric complexes with a protected reactivity cavity at the coordinated metal center. Complexes of TIMEN^R with Cu(I)/(II), Ni(0)/(I), and Co(I)/(II)/(III) have been synthesized. The cobalt(I) complexes with the aryl-substituted TIMEN^R (R = mesityl, xylyl) ligands show great potential for small molecule activation. These complexes activate for instance dioxygen to form cobalt(III) peroxo complexes that, upon reaction with electrophilic organic substrates, transfer an oxygen atom. The cobalt(I) complexes are also precursors for terminal cobalt(III) imido complexes. These imido complexes were found to undergo unprecedented intra-molecular imido insertion reactions to form cobalt(II) imine species. The molecular and electronic structures of some representative metal NHC complexes as well as the nature of the metal–carbene bond of these metal NHC complexes was elucidated by X-ray and DFT computational methods and are discussed briefly. In contrast to the common assumption that NHCs are pure σ-donors, our studies revealed non-negligible and even significant π-backbonding in electron-rich metal NHC complexes.     

Synthesis and catalytic properties of cationic palladium(II) and rhodium(I) complexes bearing diphosphinidinecyclobutene ligands

Cationic palladium(II) and rhodium(I) complexes bearing 1,2-diaryl-3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]cyclobutene ligands (DPCB–Y) were prepared and their structures and catalytic activity were examined (aryl = phenyl (DPCB), 4-methoxyphenyl (DPCB–OMe), 4-(trifluoromethyl)phenyl (DPCB–CF₃)). The palladium complexes [Pd(MeCN)₂(DPCB–Y)]X₂ (X = OTf, BF₄, BAr₄ (Ar = 3,5-bis(trifluoromethyl)phenyl)) were prepared by the reactions of DPCB–Y with [Pd(MeCN)₄]X₂, which were generated from Pd(OAc)₂ and HX in MeCN. On the other hand, the rhodium complexes [Rh(MeCN)₂(DPCB–Y)]OTf were prepared by the treatment of [Rh(μ-Cl)(cyclooctene)₂]₂ with DPCB–Y in CH₂Cl₂, followed by treatment with AgOTf in the presence of MeCN. The cationic complexes catalyzed conjugate addition of benzyl carbamate to α,β-unsaturated ketones.     

Palladium and platinum complexes with planar chiral 1,2-disubstituted ferrocenes containing phosphine and thioether donor groups

Three palladium(II) complexes and four platinum(II) complexes having general formula CpFe{1,2-C₅H₃(PPh₂)(CH₂SR)}MCl₂ (M = Pd, R = Ph, Et and tBu; M = Pt, R = Ph, Et, tBu and Cy) have been synthesized by reaction of the corresponding CpFe{1,2-C₅H₃(PPh₂)(CH₂SR)} ligands with PdCl₂(CH₃CN)₂ or PtCl₂(CH₃CN)₂. These complexes have been fully characterized in solution and in solid state. In all cases, monomeric square planar complexes were obtained as pure diastereoisomers.     

The effect of bridgehead steric bulk on the ground state and intramolecular exchange processes of (μ-SCH2CR2CH2S)[Fe(CO)3][Fe(CO)2L] complexes

The flexibility of the coordination sphere in the diiron organometallic is likely an important design component in nature's electrocatalyst for proton reduction or H₂ oxidation, i.e, the active site of [FeFe]hydrogenase. A series of complexes, (μ-SCH₂CRR′CH₂S)[Fe(CO)₃][Fe(CO)₂L] with steric bulk incorporated into the μ-S-to-S linker was synthesized and the compounds were analyzed by infrared spectroscopy and cyclic voltammetry [(R/R′ = Me/Me, Et/Et, Bu/Et), (L = CO, PPh₃, IMes (1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene), and IMe (1,3-dimethylimidazole-2-ylidene))]. While added steric bulk at the bridgehead carbon of the μ-SCH₂CR₂CH₂S produced little change in the ground state structures (X-ray diffraction) and electronic character for the (μ-SRS)[Fe(CO)₃]₂ complexes, monosubstitution of a CO with L produced distortions consistent with steric interference of the μ-SRS with nearby ligands as compared to the similar (μ-pdt)[Fe(CO)₃][Fe(CO)₂L] (pdt = S(CH₂)₃S). Variable temperature NMR studies have shown that the activation barrier for CO site exchange on the sterically bulky complexes decreases in a manner predicted by theory [J.W. Tye, M.B. Hall, M.Y. Darensbourg, Inorg. Chem. 45 (2006) 1552].     

Phosphino-(α-sulfinylalkyl)phosphonium ylide complexes (Rh,Pd) with a configurationally stable asymmetric ylidic carbon

The synthesis and structure of Rh(I) and Pd(II) complexes of chiral P,C-chelating phosphino-(α-sulfinylalkyl)phosphonium ylide ligands with a trisubstituted asymmetric ylidic center P⁺–C1R(S1(O)p-Tol)–M (R = alkyl group) have been investigated, and compared to those of the analogous disubstituted ylide complexes (R = H). Reaction of the ethyl onium ylide of o-bis(diphenylphosphino)benzene with (?)-menthyl-(S)-p-tolylsulfinate afforded the corresponding racemic erythro phosphino-(α-sulfinylethyl)phosphonium in 90% de (R = Me). The racemization process is interpreted by a Berry-like pseudorotation mechanism driven by the steric repulsion between the α-methyl substituent and the bulky menthyloxy S-substituent or sulfur lone pair in the intermediate ylide-sulfinyl adduct. The ylide of phosphino-(α-sulfinylethyl)phosphonium reacts with [Rh(cod)₂][PF₆] and PdCl₂(MeCN)₂ to afford the corresponding P,C1-chelated threo-Rh(I) and erythro-Pd(II) mononuclear complexes in 70% yield and total diastereoselectivity. These respective complexes act as efficient catalytic precursors for the hydrogenation of (Z)-α-acetamidocinnamic acid and allylic substitution of 3-acetoxy-1,3-diphenyl-1-propene with sodium dimethyl malonate. The bonding features of the erythro-Pd(II) complex exhibiting a sulfinyl O?Pd interaction are studied theoretically at the DFT level using ELF and MESP analyses. The η²-P,C haptomeric form of the ylide ligand is estimated to compete at 19% with the η¹-C haptomeric form dominating at 81%.     

DFT study of group 8 catalysts for the hydrophenylation of ethylene: Influence of ancillary ligands and metal identity

Computational methods are used to investigate catalytic hydrophenylation of ethylene using complexes of the type [(Y)M(L)(CH₃)(NCMe)]ⁿ⁺ [Y = Mp, n = 1; Y = Tp, n = 0; M = Ru or Os; L = PMe₃, PF₃, or CO; Mp = tris(pyrazolyl)methane; Tp = hydrido-tris(pyrazolyl)borate]. The conversion of ethylene and benzene to ethylbenzene with [(Y)M(L)(Ph)]ⁿ⁺ as catalyst involves four steps: (1) ethylene coordination, (2) ethylene insertion into the M–Ph bond, (3) benzene coordination, and (4) benzene C–H activation. DFT calculations form the basis to compare stoichiometric benzene C–H activation by [(Y)M(L)(CH₃)(NCMe)]ⁿ⁺ complexes to yield methane and [(Y)M(L)(Ph)(NCMe)]ⁿ⁺. In addition, starting from the 16-electron species [(Y)M(L)(Ph)]ⁿ⁺, potential energy surfaces for the formation of ethylbenzene are calculated to reveal the impact of modifications to the scorpionate ligand (Mp or Tp), co-ligand (L) and metal center (M).     

Versatile Ru-based metathesis catalysts designed for both homogeneous and heterogeneous processes

The synthesis of new ruthenium-based catalysts applicable for both homogeneous and heterogeneous metathesis is described. Starting from the Hoveyda-Grubbs first generation (1) and the Hoveyda-Grubbs second generation (2) catalysts the homogeneous catalysts [RuCl((RO)₃Si–C₃H₆–N(R′)–CO–C₃F₆–COO)(CH–o-O–iPr–C₆H₄)(SIMes)] (4: R = Et, R′ = H; 5: R = R′ = Me) (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) were prepared by substitution of one chloride ligand with trialkoxysilyl functionalized silver carboxylates (RO)₃Si–C₃H₆–N(R′)–CO–C₃F₆–COOAg (3a: R = Et, R′ = H; 3b: R = R′ = Me). These homogeneous ruthenium-species are among a few known examples with mixed anionic ligands. Exchange of both chloride ligands afforded the catalysts [Ru((RO)₃Si–C₃H₆–N(R′)–CO–C₃F₆–COO)(CH–o-O–iPr–C₆H₄)(SIMes)] (9: R = Et, R′ = H; 11: R = R′ = Me) and [Ru((RO)₃Si–C₃H₆–N(R′)–CO–C₃F₆–COO)(CH–o-O–iPr–C₆H₄)(PCy₃)] (8: R = Et, R′ = H; 10: R = R′ = Me). The reactivity of the new complexes was tested in homogeneous ring-closing metathesis (RCM) of N,N-diallyl-p-toluenesulfonamide and TONs of up to 5000 were achieved. Heterogeneous catalysts were obtained by reaction of 4, 5 and 8–11 with silica gel (SG-60). The resultant supported catalysts 4a, 5a, 8a–11a showed reduced activity compared to their homogenous analogues, but rival the activity of similar heterogeneous systems.     

Transition metal complexes based on thiophene-dithiolene ligands

The chemistry of transition metal dithiolene complexes based on thiophene-dithiolene ligands (TD) is reviewed, from the ligand synthesis and complex preparation to the molecular structure and solid state physical properties of different compounds based on them. The ligands considered are based mainly either on simple thiophene-dithiolates (α-tpdt = 2,3-thiophenedithiolate, dtpdt = 4,5-dihydro-2,3-thiophenedithiolate, and tpdt = 3,4-thiophenedithiolate), or in more extended and delocalised dithiolate ligands (α-tdt = 3-({5-[(2-cyanoethyl)thio]-2-thieno[2,3-d][1,3]dithiol-2-ylidene-1,3-dithiol-4-yl}thio)propanenitrile and dtdt = 3-{5-[(2-cyanoethyl)thio]-2-(5,6-dihydrothieno[2,3-d][1,3]dithiol-2-ylidene-1,3-dithiol-4-yl)thio}propanenitrile) that besides the thiophenic ring also incorporates a fused TTF moiety. Dithiolene complexes based on ligands containing appended thiophenic units will also be briefly considered. The structural variability of these complexes that in addition to the usual square planar coordination geometry, M(TD)₂, can also present dimeric, [M(TD)₂]₂, or cluster structures such as [Cu₄(TD)₃] and [Ni₄(TD)₆], is addressed. The role of the thiophene group and its ability to enhance electronic delocalisation from the metal dithiolene core throughout the ligand and to establish solid state networks of S?S interactions is discussed. The importance of these complexes as useful building blocks to prepare molecular materials with very interesting magnetic and transport properties, ranging from metamagnets to Single Component Molecular Metals, is illustrated by different compounds based on them.     

Synthesis and characterization of thiosemicarbazone derivatives of 2-chloro-4-hydroxy-benzaldehyde and their rhenium(I) complexes

N-Thioamide thiosemicarbazone derived of 2-chloro-4-hydroxy-benzaldehyde (R = H, HL¹; R = Me, HL² and R = Ph, HL³) have been prepared and their reaction with fac-[ReX(CO)₃(CH₃CN)₂] (X = Br, Cl) in chloroform gave the adducts [ReX(CO)₃(HL)] (1a X = Cl, R = H; 1a′ X = Br, R = H; 1b X = Cl, R = CH₃; 1b′ X = Br, R = CH₃; 1c X = Cl, R = Ph; 1c′ X = Br, R = Ph) in good yield. Complexes 1a′ and 1b’ were also obtained by the reaction of HL¹ and HL³ with [ReBr(CO)₅] in toluene.All the compounds have been characterized by elemental analysis, mass spectrometry (FAB), IR and ¹H NMR spectroscopic methods. Moreover, the structures of HL², HL³ and 1a·H₂O were also established by X-ray diffraction. In 1a, the rhenium atom is coordinated by the sulphur and the azomethine nitrogen atoms, forming a five-membered chelate ring, as well as three carbonyl carbon and chloride atoms. The resulting coordination polyhedron can be described as a distorted octahedron.The study of the crystals obtained by slow evaporation of methanol and DMSO solutions of the adducts 1a′ and 1b, respectively, showed the formation of dimer structures based on rhenium(I) thiosemicarbazonates [Re₂(L¹)₂(CO)₆]·3H₂O (2a)·3H₂O and [Re₂(L²)₂(CO)₆]·(CH₃)₂SO (2b)·2(CH₃)₂SO. Amounts of these thiosemicarbazonate complexes [Re₂(L)₂(CO)₆] (2) were obtained by reaction of the corresponding free ligands with [ReCl(CO)₅] in dry toluene.In 2a·3H₂O and 2b·2(CH₃)₂SO the dimer structures are established by Re–S–Re bridges, where S is the thiolate sulphur from a N,S-bidentate thiosemicarbazonate ligand. In both structures the rhenium coordination sphere is similar; the dimers are in the same diamond Re₂S₂ face.     

Mono- and di-nuclear complexes of thiosemicarbazones with copper(I): Synthesis,spectroscopy and structures

Reactions of copper(I) halides with a series of thiosemicarbazones, namely, benzaldehyde thiosemicarbazone (R¹R²CN–NH–C(S)–NH₂, R¹ = Ph, R² = H; Hbtsc), 2-benzoylpyridine thiosemicarbazone (R¹ = Ph, R² = py; Hbpytsc), and acetone thiosemicarbazone (R¹ = R² = Me; Hactsc), in the presence of PPh₃ has formed dimeric complexes, viz. sulfur bridged [Cu₂(μ-S-Hbtsc)₂Br₂(PPh₃)₂]·2H₂O (1), iodo-bridged [Cu₂(μ-I)₂(η¹-S-Hbtsc)₂(PPh₃)₂] (2), and heterobridged [Cu₂(μ₃-S,N³-Hactsc)(η¹-Br)(μ-Br)(PPh₃)₂] (3), as well as mononuclear complexes [CuX(η¹-S-Hbpytsc)(PPh₃)₂]·CH₃CN (X = Br, 4; Cl, 5). Complexes 1, 2, 4 and 5 involve thiosemicarbazone ligands in η¹-S bonding mode while in compound 3, ligand acts in N³, S-chelation-cum-S-bridging mode (μ₃-S,N³ mode). The intermolecular interactions such as, N²H?X, HN¹H?X (X = S, Br, Cl), CH?π interactions lead to 2D networks. All the complexes have been characterized with the help of elemental analyses, IR, ¹H, and ³¹P NMR spectroscopy, and single crystal X-ray crystallography. The role of a solvent in alteration of nuclearity and bonding modes of complexes has been highlighted.     

Synthesis and asymmetric catalytic application of chiral imidazolium–phosphines derived from (1R,2R)-trans-diaminocyclohexane

Reaction between a chiral imidazole–amine precursor derived from (1R,2R)-trans-diaminocyclohexane and P¹Cl (where P¹ = PPh₂, P(1,3,5-Me₃C₆H₃)₂, P(2,2′-O,O′-(1,1′-biphenyl), P((R)-(2,2′-O,O′-(1,1′-binaphthyl))) and P((S)-(2,2′-O,O′-(1,1′-binaphthyl)))) followed by RX (where R = ⁿPr, ⁱPr, CHPh₂, X = Br; R = ⁱPr, X = I), respectively, gives a selection of chiral imidazolium–phosphine compounds. Deprotonation of the imidazolium salt gives the corresponding NHC–P ligands that can be used in metal-mediated asymmetric catalytic applications. Catalytic reactions show that NHC–P ligands give a significantly greater rate of reaction for a palladium catalysed allylic substitution reaction in comparison to analogous di-NHC or NHC–imine ligands and that NHC–P hybrids are also effective for iridium catalysed transfer hydrogenation.     

Structure,chemical bonding,and 45Sc solid state NMR of Sc2RuSi2

The silicide Sc₂RuSi₂ was synthesized from the elements by arc-melting. The structure was refined on the basis of single crystal X-ray diffractometer data: Zr₂CoSi₂ type, C2/m, a = 1004.7 (2), b = 406.8 (1), c = 946.6 (2) pm, β = 117.95 (2), wR2 = 0.0230, 743 F² values, and 32 variables. The structure consists of a rigid three-dimensional [RuSi₂] network in which the two crystallographically independent scandium atoms fill larger cages of coordination numbers 16 and 15, respectively. The [RuSi₂] network shows short Ru–Si distances (234–247 pm) and two different Si₂ pairs: Si1–Si1 at 247 and Si2–Si2 at 243 pm. Each silicon atom has trigonal prismatic Sc₆ (for Si2) or Sc₄Ru₂ (for Si1) coordination. These building units are condensed via common edges and faces. The various Sc–Sc distances between the prisms range from 327 to 361 pm. From electronic structure investigation within DFT, chemical bonding shows a major role of Ru–Si bonding and the presence of strong electron localization around Si–Si pairs pointing to a polyanionic silicide network [RuSi₂]^δ?. The ⁴⁵Sc MAS-NMR spectra recorded at 11.7 and 9.4 T clearly resolve the two distinct scandium sites. The large electric field gradients present at both scandium sites result in typical line shapes arising from second-order quadrupole perturbation effects.     

Reactivity of intramolecularly coordinated aluminum compounds to R3EOH (E = Sn,Si). Remarkable migration of N,C,N and O,C,O pincer ligands

The reaction of organoaluminum compounds containing O,C,O or N,C,N chelating (so called pincer) ligands [2,6-(YCH₂)₂C₆H₃]AlⁱBu₂ (Y = MeO 1, ^tBuO 2, Me₂N 3) with R₃SnOH (R = Ph or Me) gives tetraorganotin complexes [2,6-(YCH₂)₂C₆H₃]SnR₃ (Y = MeO, R = Ph 4, Y = MeO, R = Me 5; Y = ^tBuO, R = Ph 6, Y = ^tBuO, R = Me 7; Y = Me₂N, R = Ph 8, Y = Me₂N, R = Me 9) as the result of migration of O,C,O or N,C,N pincer ligands from aluminum to tin atom. Reaction of 1 and 2 with (ⁿBu₃Sn)₂O proceeded in similar fashion resulting in 10 and 11 ([2,6-(YCH₂)₂C₆H₃]SnⁿBu₃, Y = MeO 10; Y = ^tBuO 11) in mixture with ⁿBu₃SnⁱBu. The reaction 1 and 3 with 2 equiv. of Ph₃SiOH followed another reaction path and ([2,6-(YCH₂)₂C₆H₃]Al(OSiPh₃)₂, Y = MeO 12, Me₂N 13) were observed as the products of alkane elimination. The organotin derivatives 4–11 were characterized by the help of elemental analysis, ESI-MS technique, ¹H, ¹³C, ¹¹⁹Sn NMR spectroscopy and in the case 6 and 8 by single crystal X-ray diffraction (XRD). Compounds 12 and 13 were identified using elemental analysis,¹H, ¹³C, ²⁹Si NMR and IR spectroscopy.     

Rhizopus arrhizus-mediated asymmetric reduction of arylalkanones: unusual anti-Prelong products with benzyl alkyl ketones

Rhizopus arrhizus-mediated microbial reduction of various aryl alkyl ketones afforded chiral carbinols in good yields and high enantiomeric purity. The most striking feature was the formation of the anti-Prelog (R)-alcohols with the benzyl alkyl ketones, while the other ketones ArXCOR (X = (CH₂)_n, n = 0 or 2, OCH₂ or SCH₂ and R = Me/Et/n-Bu) furnished (S)-alcohols.     

Enol ethers and acetals: acylation with dichloroacetyl,acetyl and benzoyl chloride in ionic liquid medium

Synthesis of 4-alkoxy-1,1-dichloro-3-alken-2-ones [CHCl₂C(O)C(R²)C(R¹)-OR, where R, R¹, R² = Et, H, H; Me, Me, H; Et, H, Me; Me, –(CH₂)₂–; Me, –(CH₂)₃–; Et, Et, H; Et, Bu, H; Et, i-Pr, H; Et, i-Bu, H; Me, Ph, H; Me, thien-2-yl, H] from acylation of enol ethers and acetals with dichloroacetyl chloride, in ionic liquid ([BMIM][BF₄] or [BMIM][PF₆]) is reported. The synthesis of alkenones [R³–C(O)C(R²)C(R¹)-OR], where R/R¹/R²/R³ = Et/H/H/Ph, t-Bu/H/H/Ph, Me/-(CH₂)₄/Ph, Me/-(CH₂)₄/Me] from the reaction of enol ethers with benzoyl chloride or acetyl chloride, in ionic liquid [BMIM][BF₄], is also reported. Last products are described for the first time.     

Enantioselective 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds catalyzed by rhodium(I)-chiral phosphoramidite complexes

A chiral bidentate phosphoramidite (5a) was synthesized from Shibasaki’s linked-(R)-BINOL and P(NMe₂)₃ as a new ligand for rhodium(I)-catalyzed asymmetric 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds. The effects of 5a and Feringa’s monodentate phosphoramidite (4, R¹, R² = Et) on the yields and enantioselectivities were fully investigated. The reaction was significantly accelerated in the presence of a base such as KOH and Et₃N, allowing the reaction to be completed at the lower temperatures than 50 °C. The addition to cyclic enones such as 2-cyclopentenone, 2-cyclohexenone and 2-cycloheptenone at 50 °C in the presence of an [Rh(coe)₂Cl]₂-4 (R¹, R² = Et) complex resulted in enantioselectivities up to 98%, though it was less effective for acyclic enones (0–70% ee). On the other hand, a complex between [Rh(nbd)₂]BF₄ and 5a completed the addition to cyclic enones within 2 h at room temperature in the presence of Et₃N with 86–99% yields and 96–99.8% ee. This catalyst was also effective for acyclic enones, resulting in 62–98% yields and 66–94% ee. The 1,4-additions of arylboronic acids to unsaturated lactones and acyclic esters with rhodium(I)-phosphoramidites complexes were also investigated.     

Activation process of 3-alkyl-substituted ansa-bis(indenyl) zirconocenes by MAO

The ansa-indene compound {1-Me₂Si(3-C₉H₆Et)₂} (1) was prepared by alkylation of the unsubstituted ansa-indene. This compound was converted, by reaction with nBuLi, to the dilithium compound [Li₂{1-Me₂Si(3-C₉H₅Et)₂}] (2). ansa-Zirconocene [Zr{1-Me₂Si(3-η⁵-C₉H₅Et)₂}Cl₂] (3) was prepared by the reaction of ZrCl₄ with 2 in ether/toluene at −78 °C. The molecular structure of meso-3 was determined by single crystal X-ray diffraction studies. The ansa-zirconocene 3 exhibits a greater activity in ethylene polymerization than reference complexes such as [Zr{1-Me₂Si(η⁵-C₉H₆)₂}Cl₂] and [Zr{1-C₂H₄(η⁵-C₉H₅)₂}Cl₂] and, in addition, it maintained a reasonable level of activity after 12 h of contact with MAO solution. Furthermore, the different elementary steps in the activation process of ethylene polymerization for substituted complexes [Zr{1-Me₂Si(3-η⁵-C₉H₅R)₂}Cl₂] (R = Et 3, Me 4, nPr 5 and nBu 6) by commercial methylaluminoxane (MAO) have been studied by UV–vis spectroscopy. Addition of MAO in large excess ([Al]/[Zr] = 2000) at −78 °C yields a previously unreported intermediate in the activation process of metallocenes; this intermediate has an absorption band centered at λ = 639 nm. We report here the influence of the type of catalyst, ring substitution, type of cocatalyst and addition of THF on the activation process of these metallocenes.     

Hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands: Imidazole and oxazole ring Lewis basicity,Ni(II)/Pd(II) complex structures and spectra

Fourteen new organic molecules A1–A4, B1–B5, C1–C4 and D and a series of transition metal(II) complexes (Ni1–Ni9 and Pd1–Pd2b) were synthesized and studied in order to characterize the hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands (A1–A4 = 2-R²-6-(4,5-diphenyl-1R¹-imidazol-2-yl)pyridines, R¹ = H or CH₃, R² = H or CH₃; B1–B5 = 1-R²-2-(pyridin-2-yl)-1R¹-phenanthro[9,10-d]imidazoles/oxazoles, R¹ = H or CH₃, R² = H or CH₃; C1–C4 = 2-(6-R²-pyridin-2-yl)-1H-imidazo/oxazo[4,5-f][1,10]phenanthrolines, R² = H or CH₃; D = 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline). They were also used to study the substituent effects on the donor strengths as well as the coordination chemistries of the imidazole/oxazole fragments of the hemilabile ligands.All the observed protonation–deprotonation processes found within pH 1–14 media pertain to the imidazole or oxazole rings rather than the pyridyl Lewis bases. The donor characteristics of the imidazole/oxazole ring can be estimated by spectroscopic methods regardless of the presence of other strong N donor fragments. The oxazoles possessed notably lower donor strengths than the imidazoles. The electron-withdrawing influence and capacity to hinder the azole base donor strength of 4,5-azole substituents were found to be in the order phenanthrenyl (B series) > 4,5-diphenyl (A series) > phenanthrolinyl (C series). An X-ray structure of Ni5b gave evidence for solvent induced ligand reconstitution while the structure of Pd2b provided evidence for solvent induced metal–ligand bond disconnection.Interestingly, alkylation of 1H-imidazoles did not necessarily produce the anticipated push of electron density to the donor nitrogen. Furthermore, substituents on the 4,5-carbons of the azole ring were more important for tuning donor strength of the azole base. DFT calculations were employed to investigate the observed trends. It is believed that the information provided on substituent effects and trends in this family of ligands will be useful in the rational design and synthesis of desired azole-containing chelate ligands, tuning of donor properties and application of this family of ligands in inorganic architectural designs, template-directed coordination polymer preparations, mixed-ligand inorganic self-assemblies, etc.     

Ortho-metalation of pyridines by cationic yttrium methyl complexes

Selective C-H bond activation of pyridines by organometallic complexes is a reaction of synthetic importance for the synthesis of functionalized pyridines. By reacting the dicationic methyl complex of yttrium [YMe(thf)₆][BPh₄]₂ with substituted pyridines, both selectivity and kinetics have been studied. Electron donating properties of para-substituents of pyridines increase the rates of reaction. Hammett linear free energy relationship was found with ρ = –2.72. DFT calculations confirmed the two-step reaction consisting of ligand substitution followed by σ-bond metathesis. DFT calculations furthermore revealed for the C-H bond activation step an unusual transition state structure with a nearly linear methyl carbon-hydrogen-ortho-carbon arrangement.     

Synthesis,characterization, bioactivities of copper complexes with N-allyl di(picolyl)amine

Four copper(II) complexes with N-allyl di(picolyl)amine were synthesized and characterized by physico-chemical and spectroscopic methods. The spectrophotometric and fluorescence titration data indicate that the [(Aldpa)Cu(L)](ClO₄)₂ (L = dppz, dione, phen) with conjugated aromatic rings as coordinated ligands can be inserted into the base stacks of DNA more deeply than the [(Aldpa)CuCl₂]. The copper(II) complexes [(Aldpa)Cu(L)](ClO₄)₂ (L = dppz, dione, phen) can inhibit the proliferation of the four cancer cells (Mcf-7, Eca-109, A549 and HeLa) with IC₅₀ 0.5–19.2 μM, which is larger than that (23.2–84.3 μM) of [(Aldpa)CuCl₂], suggesting their inhibiting activities on the four cancer cells are correlated with their DNA-binding properties. However, the selectivity of [(Aldpa)CuCl₂] to cancer cells is better than that of the other three complexes [(Aldpa)Cu(L)](ClO₄)₂, which indicates the substituents introduced on the secondary amino nitrogen atom of dpa have great contribution to the antitumor activities of these copper(II) complexes.