A theoretical study of co insertion reactions: an assessment of electron correlation effects

MP2 and CAS SCF calculations for CO insertion reactions of the type RMCO → M(COR), R = H, CH₃; M = Mn, Pd⁺ are reported. Non-dynamical correlation appears to be quite important, involving essentially the metal-carbon π bond in the Mn(I) systems and the metal-carbon and metal-hydrogen σ bonds in the Pd(II) systems.     

Group 10 metal complexes of meso-aryl-substituted [26]hexaphyrins with a metal-carbon bond

Nickel(II), palladium(II), and platinum(II) complexes of meso-aryl-substituted [26]hexaphyrin have been prepared and structurally characterized. In mono-metalated complexes, the metal ion is commonly bound with three pyrrolic nitrogen atoms and one pyrrolic beta-carbon, whereas a bis-Pd(II) complex exhibits a unique structure involving a Pd-C(alpha) bond. These results reveal novel coordination abilities of the hexaphyrin that lead to a metal-carbon bond at either pyrrolic C(beta) or C(alpha) position.     

Organometallic reactions as tools to build new metal-containing conjugated polymers

By using organometallic reactions like Pd-catalyzed C-C coupling, metal-carbon bond formation and silicon-carbon bond cleavage, novel carbon-rich organometallic monomers HC≡C-C₆H₄-C≡C-[M]-C≡C-C₆H₄-C≡CH ( [M] = -Ru(dppe)₂- and (η⁵-C₅H₄)₂Fe) and organic monomers H-(C≡C-C₆H₄)_X-C≡CH (x = 1 to 3) have been obtained. They have been used for the design of novel homo and hetero metal-containing polymers via organometallic polycondensation reactions based on quantitative metal-carbon bond formation.     

The First Titration Calorimetric Determination of Iron-Carbon Bond Dissociation Energy

Organometallic Chemistry is one of the most rapid growing areas in modern chemistry in the past a few decades. Interest in deepening a more fundamental understanding of the fabulous chemistries in this exciting area has led us to explore the information regarding to the M-C bonding energetics where M represents a metal atom, since breaking or forming a metal-carbon bond is an integral part of many key steps in organometallic reactions. Although scientists have a high desire for and have actually made considerable efforts determining metal-carbon bond strengths, the bond dissociation energy data of metal-carbon bond are still very scarce^[1-3].     

Catalytic coupling of haloolefins with anilides

A strategy in which C-H activation reactions promoted by Pd(II) have been combined with beta-heteroatom elimination to create a catalytic cycle achieving the arylation of haloacrylates is reported. The catalytic cycle can be subdivided into four parts: (1) C-H activation; (2) the functionalization step, migratory insertion of the olefin into a metal-carbon bond; (3) beta-heteroatom elimination; and (4) exchange of metal halide (if X = halogen) for a less coordinating anion. In this catalytic cycle, the oxidation state of the metal does not change, and an oxidant is not required. The method is more functional group tolerant compared with the existing alkene-arene coupling methods based on electrophilic C-H activation.     

Conversions between metal-ligand multiple bond (MLMB) types: carbonyl olefination and other applications

In this Perspective, methods for the direct conversion of one type of metal-ligand multiple bond (MLMB) to another are discussed. Of particular interest, because of their potential utility in organic synthesis, are examples of direct conversion of N- and O-based MLMB to alkylidenes and alkylidynes. Recent efforts in this area from our laboratory with some background are detailed. The research from our group involves metallacycles prepared directly from an imido ligand where a metal-carbon bond can have alkylidene-like or alkyl-like properties and reactivity depending on the ancillary ligands.     

Organosilicon compounds : XIV. The cleavage of 2-(trimethylsilyl)- and 2-(trimethylstannyl)-1-methylpyrrole and -thiophene with some carbonyl-containing reagents

Cleavage of the metal-carbon bond of 2-(trimethylsilyl)- and 2-(trimethylstannyl)-1-methylpyrrole and -thiophene with several carbonyl-containing reagents has been effected and substantiates our previous tenet for the importance of bond polarity on reactivity     

Beta-elimination in the reactions of .CR1R2CR3R4X radicals with metal powders immersed in aqueous solutions

The reactions of several radicals of the type .CR1R2CR3R4X (where X = OH or NH3+) with metal powders that have been immersed in aqueous solutions were studied. The radicals were formed by radiation chemical techniques. One of the products in all these reactions is the corresponding alkene, R1R2C=CR3R4. The results are in accord with a mechanism in which the radicals react with the metals that are forming transients with metal-carbon sigma bonds. The latter transients decompose via two competing reactions: (a) heterolysis of the metal-carbon sigma bond and (b) beta-elimination of X-. Moreover, the dehalogenation of BrCH2CH2NH3+ and ClCH2(CH3)2COH by metal powders was studied. Also in these reactions, the corresponding alkene is one of the products. This result is consistent with the suggestion that, in the dehalogenation reaction, an alkyl radical is formed in the first step. This radical then reacts with the metal. Alternatively, the transients with metal-carbon sigma bonds in the dehalogenation processes might be formed via a concerted mechanism.     

Isolation of a Uranium(III)-Carbon Multiple Bond Complex

Low-valent uranium-element multiple bond complexes remain scarce, though there is burgeoning interest regarding to their bonding and reactivity. Herein, isolation of a uranium(III)-carbon double bond complex [(Cp*)₂U(CDP)](BPh₄) ( 1 ) comprising a tridentate carbodiphosphorane (CDP) was reported for the first time. Oxidation of 1 afforded the corresponding U(IV) complex [(Cp*)₂U(CDP)](BPh₄)₂ ( 2 ). The distance between U and C in 2 is 2.481 Å, indicating the existence of a typical U=C double bond, which is further confirmed by quantum chemical calculations. Bonding analysis suggested that the CDP also serves as both σ- and π-donor in complex 1 , though a longer U−C bond (2.666(3) Å) is observed. It implies that 1 is the first isolable mononuclear uranium(III) carbene complex. Moreover, these results suggest that CDPs are promising ligands to establish other low-valent f-block metal-carbon multiple bond complexes.     

Doppelkomplexsalze von Co(II), Ni(II) und Cu(II) mit Thioharnstoffliganden im Kationkomplex und Thiocyanatliganden im anionkomplex

Three new double complex compounds of the following type were obtained: [CoThio ₄][Co(SCN)₄], [NiThio ₄][Ni(SCN)₄] and (CuThio ₄) (CuCo(SCN)₄). The melting points of the compounds were determined, and the molecular weight of the first. The IR-spectra were studied and the metal-ligand bond interpreted. It was shown that the metal-thiourea bond in all compounds is formed via the sulphur atom. In the complex anion of the first and second compounds Co(II) and Ni(II) are coordinated with SCN^– through the nitrogen atom. In the third, more complicated compound, Cu(II) is coordinated to SCN^– through the sulphur atom, and Co(II) through the nitrogen atom, a bridging bond being formed.     

Synthetic and Mechanistic Aspects of Inorganic Insertion Reactions. Insertion of Carbon Monoxide

The synthetic potential and the mechanistic aspects of inorganic insertion reactions of carbon monoxide, especially into metal-carbon σ-bonds, are considered. Reactions of this type are encountered among most 3d, 4d, and 5d elements. In one case it has been demonstrated that the CO insertion proceeds by alkyl migration; this is likely to be a general feature of all such reactions. If an alkyl migration takes place, then insertion of CO into the M? C bond is governed kinetically by the cleavage of that bond. CO abstraction from RCO? M bonds most commonly proceeds by rate-determining vacation of a coordination position. Both CO insertion and abstraction are usually highly stereospecific.     

Two isomers of Pd2(S2NC7H4)4

The dichloromethane solvates of the isomers tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁴N:S;κ⁴S:N‐dipalladium(II)(Pd—Pd), (I), and tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁶N:S;κ²S:N‐dipalladium(II)(Pd—Pd), (II), both [Pd₂(C₇H₄NS₂)₄]·CH₂Cl₂, have been synthesized in the presence of (o‐isopropylphenyl)diphenylphosphane and (o‐methylphenyl)diphenylphosphane. Both isomers form a lantern‐type structure, where isomer (I) displays a regular and symmetric coordination and isomer (II) an asymmetric and distorted structure. In (I), sitting on an centre of inversion, two 1,3‐benzothiazole‐2‐thiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the other two benzothiazolethiolate units are bonded to the same Pd atoms by, respectively, a Pd—S and a Pd—N bond. In (II), three benzothiazolethiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the fourth benzothiazolethiolate unit is bonded to the same Pd atoms by, respectively, a Pd—S bond and a Pd—N bond.     

Oligonucleosides with a Nucleobase‐Including Backbone,Part 7

The conformational analysis of 7 was carried out in (D₆)DMSO and in mixtures of (D₆)DMSO and CDCl₃ to evaluate the syn/anti conformations, relevant to the pairing propensity of this type of nucleotide analogue. The HO−C(5′) of unit I and of unit II of the dimer 7 form an intramolecular H‐bond to N(3). In (D₆)DMSO, the C(5′)−OH⋅⋅⋅N(3) H‐bond in unit I is partially broken, while that in unit II persists to a larger extent. The syn conformation prevails for unit I and particularly for unit II. The furanosyl moieties adopt predominantly a 2′‐endo conformation that is largely independent of the solvent.     

Transcyclometalation,a versatile methodology for multiple metal-carbon bond formation with multisite ligands

Transcyclometalation has been successfully applied for the multi-platination and -ruthenation of 'chartwheel'-type ligand systems containing six potential metal binding sites, thus providing a method for multiple metal-carbon bond formation via C-H bond activation which is superior to established cyclometalation protocols.     

Synthesis and Characterization of a New 2,2′-Azoquinoxaline and its Complexeswith Platinum and Palladium

Summary.  Platinum and palladium dichlorides react readily with 2,2′-azoquinoxaline to give the di-μ-chloro-2,2′-azobis-(7,7′-bis-benzoyl-quinoxaline) diplatinum(II) and dipalladium(II) complexes. These crystalline complexes exhibit both metal-carbon binding and coordinate metal-nitrogene binding to azoquinoxaline. Received July 16, 1999. Accepted (revised) October 22, 1999     

Enthalpy and entropy changes associated with mixed ligand chelates of Cu(II) and Ni(II) with 4-methoxy picolinic acid N-oxide and some amino acids in aqueous medium

The stability constants of mixed complexes of Cu(II) and Ni(II) with 4-methoxy picolinic acid N-oxide, and glycine, α-alanine, proline and hydroxy-proline have been determined at various temperatures by the potentiometric method in 0·1 M ionic strength. The formation constants of the mixed complexes have been evaluated and are in good agreement with statistically expected values. The enthalpy and entropy values have been calculated from 1∶1∶1 stability constants temperature coefficient data. From the enthalpy values of the mixed complexes it may be concluded that the bond strengths are not equal to the average of the bond strengths inMA ₂ andMB ₂ type parent complexes. The entropy values have been found to be favourable for ternary complex formation.     

Alkali scandium arsenates. I. The framework structures of KSc(HAsO4)2 and RbScAs2O7

The crystal structures of hydro­thermally synthesized potassium scandium hydrogen arsenate(V), KSc(HAsO₄)₂, (I), and rubidium scandium diarsenate(V), RbScAs₂O₇, (II), were determined from single‐crystal X‐ray diffraction data collected at room temperature. Compound (I) represents a new microporous structure type, designated MCV‐3, which is characterized by a three‐dimensional framework of corner‐sharing alternating ScO₆ octahedra and HAsO₄ tetrahedra. Intersecting tunnels parallel to [101] and [110] host eight‐coordinate K atoms. There is one hydrogen bond of medium strength [O⋯O = 2.7153 (18) Å]. Compound (II) is the first reported diarsenate with a KAlP₂O₇‐type structure and is isotypic with at least 27 A^IM^III diphosphates. The average Sc—O bond lengths in (I) and (II) are 2.09 (2) and 2.09 (3) Å, respectively. The K and Sc atoms in (I) lie on an inversion centre and a twofold axis, respectively. All atoms in (II) are in general positions.     

Organometallic Tetrazole Derivatives: Preparation and Application to Organic Synthesis

The data are integrated and systematized on preparation procedures and application to the organic synthesis of organometallic tetrazole derivatives, including 5-metallated tetrazoles and tetrazoles with a metal-carbon bond in a substituent, and also organotin tetrazoles.     

Synthesis and X-ray crystal structure of cis-dichloro(cyclopentylamine) (dimethyl sulphoxide)platinum(II)

Summary cis-Dichlorocyclopentylamine(dimethyl sulphoxide)-platinum(II) is obtained, in addition to small quantities of the corresponding trans compound, by reaction of K₂PtCl₄ with cyclopentylamine in DMSO solution, where it is formed as the thermodynamically favoured isomer. An X-ray crystal structure analysis confirms the cis configuration. Coordination around the metal centre is square planar, and the ligand bond angles at the Pt atom are close to the expected values of 90 and 180°. The DMSO ligand is S-coordinated to Pt. The Pt-Cl bond lengths, 2.299(2) and 2.317(2) Å, are normal for structures of this type, as are the Pt-N and Pt-S bond distances, 2.059(5) and 2.191(2) Å, respectively.Author to whom all correspondence should be directed.     

Reconstitution of a Type II Polyketide Synthase that Catalyzes Polyene Formation

While type II polyketide synthases (PKSs) are known for producing aromatic compounds, a phylogenetically new subfamily of type II PKSs have been recently proposed to synthesize polyene structures. Here we report in vitro analysis of such a type II PKS, IgaPKS for ishigamide biosynthesis. The ketoreductase (Iga13) and dehydratase (Iga16) were shown to catalyze the reduction of a β‐keto group and dehydration of a β‐hydroxy group, respectively, to form a trans double bond. Incubation of the acyl carrier protein (Iga10), the ketosynthase/chain length factor complex (Iga11–Iga12), Iga13 and Iga16 with malonyl and hexanoyl‐CoAs and NADPH followed by KOH hydrolysis resulted in the formation of four unsaturated carboxylic acids (C₈, C₁₀, C₁₂, and C₁₄), indicating that IgaPKS catalyzes tetraene formation by repeating the cycle of condensation, keto‐reduction and dehydration with strict stereo‐specificity. We propose “highly reducing type II PKS subfamily” for the polyene‐producing type II PKSs.