Well-defined Bis Research Articles

Bis(diiminate)-based boron difluoro complexes: effective synthon for bis(borenium) cations.

A series of boron difluoro bis(diiminate) complexes have been prepared and used to obtain triflate substituted fluoroborane complexes. The corresponding well-defined bis(borenium) cations were subsequently synthesized and structurally authenticated. We are also presenting the first experimental and theoretical study of bis(borenium) cations that are derivative of cationic borinic acid.

Well-Defined Bis(NHC)Mn(I) Complex Catalyzed Tandem Transformation of α,β-Unsaturated Ketones to α-Methylated Ketones Using Methanol

Well-Defined Bis(NHC)Mn(I) Complex Catalyzed Tandem Transformation of α,β-Unsaturated Ketones to α-Methylated Ketones Using Methanol

Deoxygenation of Nitrous Oxide and Nitro Compounds Using Bis(N-Heterocyclic Silylene)Amido Iron Complexes as Catalysts.

Herein, we report the efficient degradation of N2 O with a well-defined bis(silylene)amido iron complex as catalyst. The deoxygenation of N2 O using the iron silanone complex 4 as a catalyst and pinacolborane (HBpin) as a sacrificial reagent proceeds smoothly at 50 °C to form N2 , H2 , and (pinB)2 O. Mechanistic studies suggest that the iron-silicon cooperativity is the key to this catalytic transformation, which involves N2 O activation, H atom transfer, H2 release and oxygenation of the boron sites. This approach has been further developed to enable catalytic reductions of nitro compounds, producing amino-boranes with good functional-group tolerance and excellent chemoselectivity.

C(sp2)-H Borylation of Heterocycles by Well-Defined Bis(silylene)pyridine Cobalt(III) Precatalysts: Pincer Modification, C(sp2)-H Activation and Catalytically Relevant Intermediates.

Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp2)-H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and pyridines. Specifically, [( Ar SiNSi)CoH3]·NaHBEt3 ( Ar SiNSi = 2,6-[EtNSi(NtBu)2CAr]2C5H3N, Ar = C6H5 (1-H 3 ·NaHBEt 3 ), 4-MeC6H4 (2-H 3 ·NaHBEt 3 )) and trans-[( Ar SiNSi)Co(H)2BPin] (Ar = C6H5 (1-(H) 2 BPin), 4-MeC6H4 (2-(H) 2 BPin), Pin = pinacolato) were prepared and employed as single component precatalysts for the C(sp2)-H borylation of 2-methylfuran, benzofuran and 2,6-lutidine. The cobalt(III) precursors, 2-H 3 ·NaHBEt 3 and 2-(H) 2 BPin also promoted C(sp2)-H activation of benzofuran, yielding [(ArSiNSi)CoH(Bf)2] (Ar = 4-MeC6H4, 2-H(Bf) 2 , Bf = 2-benzofuranyl). Monitoring the catalytic borylation of 2-methylfuran and 2,6-lutidine by 1H NMR spectroscopy established the trans-dihydride cobalt(III) boryl as the catalyst resting state at low substrate conversion. At higher conversion two distinct pincer modification pathways were identified, depending on the substrate and the boron source.

Synthesis and structural characterization of well-defined bis(oxamato)palladate(II) precatalysts for Suzuki and Heck reactions

Abstract A family of tetra-n-butylammonium salts of bis(oxamato)palladate(II) complexes of formula (n-Bu4N)2[Pd(Lm)2]·pH2O [m = 1–9; L1 = N-phenyloxamate (pma) and p = 2 (1), L2 = N-2-methylphenyloxamate (2-Mepma) and p = 4 (2), L3 = N-4-methylphenyloxamate (4-Mepma) and p = 2 (3), L4 = N-2,3-dimethylphenyloxamate (2,3-Me2pma) and p = 3 (4), L5 = N-2,4-dimethylphenyloxamate (Me2pma) and p = 4 (5), L6 = N-2,5-dimethylphenyloxamate (2,5-Me2pma) and p = 7 (6), L7 = N-3,4-dimethylphenyloxamate (3,4-Me2pma) and p = 6 (7), L8 = N-3,5-dimethylphenyloxamate (3,5-Me2pma) and p = 5 (8)] have been prepared and characterized by spectroscopic methods. The crystal structure of 1 that has been solved by single crystal X-ray diffraction shares main structural features with that reported for (n-Bu4N)2[Pd(L9)2]·2CH3CONH2 [L9 = N-2,4,6-dimethylphenyloxamate (2,4,6-Me3pma) and CH3CONH2 = acetamide] (see F.R. Fortea-Perez et al. (2016)). Each palladium atom in 1 is four-coordinate with two oxygen and two nitrogen atoms from two fully deprotonated pma ligands building a centrosymmetric square-planar surrounding, the Pd N/Pd O bond lengths being 2.020(3)/2.003(2) A. The bis(oxamato)palladate(II) units in 1 are well separated from each other by the bulky organic cations, the resulting value of the shortest metal–metal separation being 9.56 A. This series of very stable, phosphine-free, complexes with well-defined structures were shown to efficiently catalyze the Heck reaction of aryl halides with olefins in DMF at 80 °C in 3 h, most likely involving Pd(II)/Pd(IV) species in the vinylation reaction of aryl halides, as well as the Suzuki coupling of aryl halides with arylboronic acids in the same solvent under similar conditions. The reaction conditions for both Heck and Suzuki couplings were optimized by varying the amount of the catalyst, and their scope was systematically studied by using different olefins and aryl halides. The influence of the number and positions of the methyl substituents from the phenyloxamate ligand on the catalytic activity is analyzed and discussed in the light of the results obtained.

Tungsten(VI) Carbyne/Bis(carbene) Tautomerization Enabled by N-Donor SBA15 Surface Ligands: A Solid-State NMR and DFT Study.

Designing supported well-defined bis(carbene) complexes remains a key challenge in heterogeneous catalysis. The reaction of W(≡CtBu)(CH2 tBu)3 with amine-modified mesoporous SBA15 silica, which has vicinal silanol/silylamine pairs [(≡SiOH)(≡SiNH2 )], leads to [(≡SiNH2 -)(≡SiO-)W(≡CHtBu)(CH2 tBu)2 ] and [(≡SiNH2 -)(≡SiO-)W(=CHtBu)2 (CH2 tBu). Variable temperature, (1) H-(1) H 2D double-quantum, (1) H-(13) C HETCOR, and HETCOR with spin diffusion solid-state NMR spectroscopy demonstrate tautomerization between the alkyl alkylidyne and the bis(alkylidene) on the SBA15 surface. Such equilibrium is possible through the coordination of W to the surface [(≡Si-OH)(≡Si-NH2 )] groups, which act as a [N,O] pincer ligand. DFT calculations provide a rationalization for the surface-complex tautomerization and support the experimental results. This direct observation of such a process shows the strong similarity between molecular mechanisms in homogeneous and heterogeneous catalysis. In propane metathesis (at 150 °C), the tungsten bis(carbene) tautomer is favorable, with a turnover number (TON) of 262. It is the highest TON among all the tungsten alkyl-supported catalysts.

A Highly Chemoselective Cobalt Catalyst for the Hydrosilylation of Alkenes using Tertiary Silanes and Hydrosiloxanes

The hydrosilylation of alkene substrates bearing additional functionalities is difficult to achieve using earth-abundant catalysts and has not been extensively realized with both earth-abundant transition metals and tertiary silanes or hydrosiloxanes. Reported herein is a well-defined bis(carbene) cobalt(I)-dinitrogen complex for the efficient, catalytic anti-Markovnikov hydrosilylation of terminal alkenes, featuring a broad substrate scope. Alkenes containing hydroxyl, amino, ester, epoxide, ketone, formyl, and nitrile groups are selectively hydrosilylated in this reaction sequence. Multinuclear NMR studies of reactive intermediates gave insights into the mechanism.

Chemoselective Alkene Hydrosilylation Catalyzed by Nickel Pincer Complexes.

Chemoselective hydrosilylation of functionalized alkenes is difficult to achieve using base-metal catalysts. Reported herein is that well-defined bis(amino)amide nickel pincer complexes are efficient catalysts for anti-Markovnikov hydrosilylation of terminal alkenes with turnover frequencies of up to 83,000 per hour and turnover numbers of up to 10,000. Alkenes containing amino, ester, amido, ketone, and formyl groups are selectively hydrosilylated. A slight modification of reaction conditions allows tandem isomerization/hydrosilylation reactions of internal alkenes using these nickel catalysts.

A Well‐Defined Silica‐Supported Lanthanum Bis(phosphinimino)methanide

AbstractGrafting of the bis(phosphinimino)methanide complex [La{CH(PPh2NSiMe3)2}{N(SiHMe2)2}2] (1) onto dehydroxylated silica (SiO2‐700) affords material 2. Full characterization of this material by infrared spectroscopy, elemental analysis, and multinuclear, multidimensional solid‐state NMR spectroscopy showed formation of well‐defined bis(phosphinimino)methanide amido surface species, [(≡SiO)La{CH(PPh2NSiMe3)2}{N(SiHMe2)2}], along with [≡SiOSiHMe2] sites.

Well-defined bis(phenolate) lanthanide methoxides as efficient initiators for the polymerization of cyclic carbonate and lactide

Lanthanide methoxides supported by carbon-bridged bis(phenolate) ligands 2,2′-methylene-bis(6-tert-butyl-4-methylphenoxo) (MBMP2−) [(MBMP)Ln(µ-OMe)(THF)2]2 (Ln = Nd (1), Yb (2)) were synthesized in high yields by the protolysis reaction using (C5H5)3Ln(THF) as starting materials, and complex 1 was structurally characterized. The coordination geometry around the center metal can be best described as a distorted octahedron. Complexes 1 and 2 were shown to be efficient initiators for the ring-opening polymerization of 2,2-dimethyltrimethylene carbonate (DTC) and L-lactide. The mechanism of DTC polymerization was explored by the end group analysis of the oligomer.

Ring-opening polymerization of d, l-lactide with bis(trimethyl triazacyclohexane) praseodymium triflate

Ring-opening polymerization of d, l-lactide (LA) was catalyzed by the well-defined bis(trimethyl triazacyclohexane) praseodymium triflate [(Me 3TAC) 2Pr(OTf) 3] (Cat) for the first time. The effect of the [LA]/[Cat] molar ratio, solvents, temperature, and reaction time were investigated in detail. The results show that the praseodymium catalyst is very efficient in the melt polymerization, giving d, l-PLA with molecular weights of 10 4 in good conversion. The importance of the triazacyclohexane was shown by comparison to the poor catalytic activity of Pr(OTf) 3.

Polyimides with main-chain ethylene oxide units: synthesis and properties

This paper describes the synthesis of a new series of well-defined bis(ether anhydride)s based on ethylene oxide sequences of known length (up to six ethylene oxide units) with phthalic acid anhydride end-caps, and extends the synthesis of bisphenoxyamines based on ethylene oxide sequences to six ethylene oxide units. The dianhydrides and diamines were used in combination to synthesize a series of segmented polyimides of well-defined structures having defined sequences of from zero up to six ethylene oxide units separated by N-phenylphthalimide units. The polymers are characterized in terms of solubility, thermal stability, thermal transition behaviour, including crystal melting behaviour; glass-transition temperatures vary systematically with structure and composition. One polymer exhibits liquid crystallinity. The water absorption characteristics of the polymers, which vary from a hydrophobic aromatic polyimide to polymers containing up to 52.7 wt% ethylene oxide, were determined. The crystal melting behaviour is discussed in conjunction with data from other segmented polyimides containing similar structural units.