Search

The role of Fe in biological and industrial N2 fixation has inspired the intense study of small molecule analogues of Fe-(NxHy) intermediates of potential relevance to these processes. Although a number of low-coordinate Fe-(N2) featuring varying degrees of fidelity to the nitrogenase active site are now known, these complexes frequently feature strongly donating ligands that either enforce low- or intermediate-spin states or result in linear Fe-(N2)-Fe bridging motifs. Given that the nitrogenase active site uses weak-field sulfide ligands to stabilize its reactive Fe center(s), N2 binding to high-spin Fe is of great interest. Herein, we report the synthesis and characterization of the first terminal N2 complex of high-spin (S = 3/2) Fe(I) as well as a bridging Fe-(N2)-Fe analogue. Electron paramagnetic resonance and solution magnetic moment determination confirm the high-spin state, and vibrational experiments indicate a substantial degree of activation of the N≡N bond in these complexes. Density functional theory calculations reveal an electronic structure for the terminal adduct featuring substantial delocalization of unpaired spin onto the N2 ligand.

A study explores the N/[eta.sup.2] equilibrium for TpW(NO)-(P[Me.sub.3])(pyridine) and its dependence on the heterocycle substituents and metal oxidation state. It is found that the ability of tungsten to bind pyridines in an [eta.sup.2] manner is dependent on both the electronic and steric properties of the substituents as well as the oxidation state of the metal.

A series of metal complexes was synthesized in which arenes were dihapto-coordinated to pi-basic metal fragments having the general form {TpM(pi-acid)(L)}. The arene complexes were shown to be significantly more basic than the analogous pentaammineosmium(ll) arene complexes and were protonated by moderate acids to give remarkably stable eta2 and eta3 arenium cation complexes.

The description of a methodology to resolve TpRe(CO)(1-methylimidazole)(eta2-benzene) using (R)-alpha-pinene is presented. Each enantiomer of the {TpRe(CO)(Melm)} system is obtained by studying the different rates of pinene substitution.

The pi basic metal fragment {TpRe(CO)(Melm)} forms stable complexes with esters, amides, imides, and anhydrides, offering unusual examples of dihapto-coordinated carboxylic acid derivatives. X-ray and NMR data for the compound TpRe(CO)(Melm)(eta(super 2)-N-methylsuccinimide) indicate that metal coordination effectively removes the pi-interaction between the bound carbonyl and the nitrogen of the succinimide ring.

Research indicates that furan or alkylated furan complexation with an electron-rich rhenium complex activates the heterocycles to produce either [2+2] or [3+3] cycloaddition reactions, depending on the position of the coordinated ring. Data reveal that metal accelerates the reaction more than the organic analogues.

A series of complexes of the form TpRe(CO)(L)(eta(super 2)-naphthalene) undergoes tandem electophile/nucleophile addition reactions with a high degree of regiocontrol depending on the auxiliary ligand. The unbound ring of the naphthalenium extends toward L, nucleophilic attack at C2 allows the C=C bond of the resulting 1,2-dihydronaphthalene to be orthogonal to the Re-Co pi bonds, thus maximizing the back-bonding interaction with the dihydronaphthalene.

The addition of methanol to dihapto-coordinated rhenium compounds of furan was studied to determine reaction rate, stereochemistry and regiochemistry.

The complex TpRe(CO)2 binds with aromatic compounds and forms binuclear complexes of the form (TpRe(CO)(sub 2))(sub 2)(mu-eta(super 2)-eta(super 2')-L) where L = furan, N-methylpyrrole or naphthalene. The furan-bridged complex is the result of a transient eta(super 2)-furan monomeric complex. The transient monomeric complex could also be the reason for the formation of the two other complexes. However, the electron density at the metal center is diminished by the presence of two carbonyl ligands.

A new process for synthesizing cis-1,4-dihydronaphthalenes in good yields directly from naphthalene was developed. This requires a dearomatization process which makes use of the electron-donating characteristics of pentaammineosmium(II) to promote electrophilic addition to Cl and to stabilize the 1-naphthalenium products such that a carbon-based nucleophile may be introduced. The procedure does not need any specialized glassware.

3-alkylated anisole compounds of pentaammineosmium(II) were treated with methyl vinyl ketone and triflic acid to form 4H-anisolium Michael adducts, which deprotonate regioselectively at the benzylic position next to C3, after which they undergo an aldol cyclization. The dienonium product reduction can be directed in 1,4 way to provide a trans decalin, or in a 1,2 way to provide methoxydiene.

Complexation of an arene by an electron-deficient transition metal center has been much researched over the past 30 years. Anisoles were complexed by pentaammineosmium(II) and the result 5,6-eta2-anisole compounds were treated with activated olefins or acetals, forming 4H-anisolium compounds. They were able to undergo inter or intramolecular nucleophilic addition reactions, and 2-methoxy-1,3-cyclohexadiene complexes were formed. These can be readily converted into functionalized cyclohexenones, cyclohexadienes, and cyclohexenes.

Eta2-arene complexes (2a-1) of pentaammineosmium were prepared in which the alkoxy substituent R1 bears a potential hydrogen bond acceptor. A two-point interaction was achieved between the pentaammineosmium system and the organic ligand. Complex 2a underwent stereospecific protonation to form a 4H-anisolium product 3a while complex 3a underwent nucleophilic addition of a silylketene acetal to form an alkaoxydiene complex (4) as a single diastereometer.

A study was conducted on the reaction of the complex of [Os(NH3)5(eta2-anisole)]2+ with acetals or Michael acceptors to produce 4-methoxystyrene complexes that are exclusively coordinated at the arene (eta2). These styrene complexes are active dienes and are involved in Diels-Adler reactions with electron-deficient olefins to form tetrahydronaphthalene complexes. The cycloadducts are produced as single diastereomers and are essential as precursors to functionalized tetralins and decalins.

An experiment evaluated the basic transmutations of eta2-coordinated furan complexes of pentaammineosmium(II). Coordination of the electron-rich pentaammineosmium(II) moiety to a furan significantly changes the structural and chemical character of the ligand, with the generated complexed ligands easily experiencing different electrophilic addition reactions that feature two reactive species. These species are the apparently interchangeable open-chain eta2-vinyl cation species and eta2-3H-furanium species.

A series of eta2-aldehyde complexes of pentaammineosmium(II) of the form (Os(NH3)5(L))(OTf)2 (L = formaldehyde, acetaldehyde, trimethylacetaldehyde, benzaldehyde and crotonaldehyde, OTf = trifluoromethanesulfonate) were synthesized. Treatment of these eta2-bound aldehyde complexes with CH3OTf resulted into Fischer carbyne complexes. Protonation or methylation of many of these aldehyde complexes produced O-protonated or O-methylated eta2-aldehydium intermediates.

The electrophile-promoted carbon-sulfur bond cleavage in Eta2-thiophene complexes of pentaammineosmium(II) was studied. S-alkylthiophenum complexes were prepared using alkylation of thiophene complexes. Results show that the nucleophile S-alkylthiophenum undergo cleavage of the C5-S bond to produce Eta4-4-(alkylthio)-1,3-butadiene complexes.

1H and 13C nuclear magnetic spectrometry were done to study the protonation of complexed anisoles and that of benzene, alkylated benzenes and polyaromatic hydrocarbons bound to osmium(II). The results indicated that the metal confers stability to the hydrocarbon arenium system through its coordination with the organic ligand in an eta-3 mode. In cases of unactivated arene or polyaromatic hydrocarbon, the arenium ion was found to be dihapto-coordinated, with the corresponding anisolium system exhibiting eta-3 coordination.

The synthesis of a series of 4,5-eta2-Os(II)pentaammine-3-vinylpyrrole complexes from the corresponding 1-methylpyrrole complex by an electrophilic addition at the beta-carbon of the pyrrole ring is described. The process involved four independent synthetic routes to beta-vinylpyrrole complexes, with each introducing a different regioselective beta-electrophilic addition for a dihapto-coordinated pyrrole. With isomerization quite slow, Diels-Alder reactions can be undertaken with electron-deficient olefins. The tetrahydroindole complexes generated are easily convertible to the corresponding indoles.

The nucleophilic substitution reactions of olefins bound to the pi-base pentaammineosmium(II) were investigated. A series of eta2-(Os NH3 sub5 vinyl ether)2+ complexes was prepared by three independent methods that involves nucleophilic substitution of an eta2-vinyl ether species, alcohol addition to an eta2-alkyne complex or direct coordination of a vinyl ether. The results showed that the substitution involves the electrophile-induced removal of the leaving group followed by nucleophilic addition to a vinyl cation intermediate.

Complexes of alkylated phenols with pentaammineosmium(II) react with various Michael acceptors in acidic and basic medium to produce 2,4- or 2, 5-cyclohexadien-1-one complexes. Reaction conditions and nature of the electrophile influence the substitution at C4, while selective addition takes place at C4 and C6. Heating removes the metal and rearomatizes the ligand in cyclohexadienone having an sp3 methine carbon while oxidation removes the metal in dienones having quaternary sp3 carbon.

The phenol-dienone equilibrium for a series of eta2-phenol complexes of pentaamineosmium(II) was examined to investigate the degree of dearomatization in these systems. The complexes with the form (Os(NH3)5(2, 3-eta2-phenol))2+ were characterized by 1H NMR spectroscopy and compared with an anisole derivative. Protonation and deprotonation were favored at the ring face opposite the metal accounting for the stereoselectivity of the tautomerization processes.

A combination of x-ray crystallography and various other techniques is employed to explain the synthesis, structure and properties exhibited by the family of trigonal pyramidal iron(II) pyrrolide complexes. The static and dynamic magnetic behavior of the compounds is shown to be highly dependent on the applied ligand field.

The N-acetylpyridinium complex of {TpW(NO)(P[Me.sub.3])} has undergone regio- and stereoselective reactions with a broad range of common organic nucleophiles, which as provided a family of 1,2-dihydropyridine (DHP) complexes of the form TpW(NO)(P[Me.sub.3])(3,4-[[eta].sup.2]DHP). The subsequent oxidative decomplexation of the materials has produced many piperidinamides with unconventional substitution patterns.

Slow magnetic relaxation is observed for a trigonal pyramidal complex of high-spin iron(II) ([[([tpa.sup.Mes])Fe].sup.-], which has provided the first example of a mononuclear transition metal complex that has behaved as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements have shown strong uniaxial magnetic anisotropy acting on the ground state of the molecule and ac magnetic susceptibility measurements have shown the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization.

An experiment has shown that beta-hydride elimination can be used for the direct transition of a coordinated amine to an iminium hydride. The microscopic reverse process of the iminium hydride into a metal-hydride bond has also been reported.

The studies have shown that a borane adduct is an effective synthon for the preparation of [pi]-complexes of basic N-heterocycles. The dearomatized acetylpyridinium complex regio- and stereoselectivity is combined with mild carbon nucleophiles to give C2-substituted dihydropyridine complexes that are elaborated into highly functionalized piperidines.

The complex TpW(NO)(P[Me.sub.3])([[eta.sup.2]-anisole) is combined with acrolein or methyl vinyl ketone and various nucleophiles in order to generate novel chromen complexes. The complexes are elaborated by protonation and nucleophilic addition to generate chroman analogues with increased saturation and stereocenters and the treatment with various oxidants has affected the decomplexation of the chromen.

Several studies are conducted to analyze the stereoselective umpolung tandem addition of different heteroatoms to phenol. The reaction sequence is shown to be highly versatile and hence can be easily employed for the formation of new functionalized cyclohexenones.

A new non-heme iron pyrrole platform and its ability to activate [N.sub.2]O for intra- and intermolecular oxidation reactions are described. The studies have shown a rare heterogeneous system that is capable of mediating oxidative O-atom transfer from [N.sub.2]O.

The [2 + 2] cycloaddition reaction of ketenes with alkenes to form cyclobutanones, which have not depended on an enolate(phenolate) intermediate, is tested. The [[eta].sup.2]-complexation of phenol has displayed an isolated alkene fragment of the 2H-phenol tautomer, which can undergo a regio- and stereoselective [2 + 2] cycloaddition with ketenes of DMAD.

A fundamentally new methodology that constructs the isoquinuclidine core directly from pyridines, using a pi-basic tungsten complex to disrupt the aromatic stabilization of the otherwise inert heterocycles is reported. Common pyridines are found to undergo stereoselective Diels-Alder reactions with election-deficient alkenes under mild reaction conditions, thus providing access to a broad range of functionalized isoquinuclidines.