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The elimination-addition mechanism for nucleophilic substitution at the vinylic carbon is established, and the reaction can be employed as a mild method for generation of cyclohexynes. The regioselectivity of the nucleophilic addition to the substituted cyclohexynes is explored.

The heterolytic cleavage of H2 is regarded as a key step in the metalloenzymatic reactions of hydrogenases, the actives sites of which consist of the sulfur-bridged Fe/Fe and Fe/Ni dinuclear centers. Based on the experimental results, the mechanism of the heterocyclic H2 activation and the reverse H2 evolution reactions are discussed.

Half-sandwich iron complexes of N-heterocyclic carbenes are prepared by the reaction of Cp*Fe{N[(Si[Me.sub.3]).sub.2]} with the corresponding imidazolium salts. Coupling reactions between heteroarenes and catecholborane (HBcat) is carried out by treatment of cyclometalated complex with heteroarenes followed by the addition of excess HBcat, giving rise to 2-boryl-heteroarenes and the borohydride complex Cp*Fe([L.sup.[super i][P.sub.r]])([H.sub.2]Bcat).

The 2,6-dimesitylphenyl thiolate (SDmp) complex of iridium has promoted facile [H.sub.2] heterolysis generating [Cp*Ir(P[Me.sub.3])[H.sub.3]](B[Ar.sub.4.sup.F]) and H-SDmp. The kinetic isotope effect has shown that an H-H cleaving process is involved in the rate-determining step and it is compare with the one estimated for the [H.sub.2] activation by the [NiFe] hydrogenase from Allochromatium vinosun.

A coordinatively unsaturated dinuclear iron(II) complex of bulky thiolates is synthesized from stepwise reactions of Fe[{N[(Si[Me.sub.3]).sub.2]}.sub.2], HSDmp, HSTip and elemental sulfur (where Tip = 2,4,6-[super i][Pr.sub.3][C.sub.6][H.sub.2] and Dmp = 2,6-[(mesityl).sub.2][C.sub.6][H.sub.3]). The structural relevance of [8Fe-7S] clusters to P-cluster, FeMo-co and FeFe-co of nitrogenases is described.

The structure of the inorganic core of P-cluster which can be generated by a self-assembly reaction of Fe(II) bis-amide, tetramethylthiourea, 2,4,6-triisopropylbenzenethiol, and elemental sulfur (S8) is reported. Oxidation of the P(super 2+) state is reported to produce a cluster with Fe(II)(sub 6) and Fe(III)(sub 2).

The article reports on the reversible heterolysis of H2 arbitrated by an M-S(Thiolate) bond(M=Ir,Rh). It discusses that heterolytic segmentation of dihydrogen is a fundamental pace in the purpose of hydrogenases, which catalyze hydrogen development and consumption under mild conditions. It implies that various conversion metal complexes have been shown to promote heterolysis of H2.

The article reports on the synthesis of nickel complexes. Hydrogenase is essential to hydrogen metabolism by catalyzing the reversible interconversion of proton and molecular hydrogen. The enzyme has been found in many microorganisms of biotechnological interest, which can be grouped into two classes based on the type of metal-containing active sites, namely, [Fe]-only and [NiFe] hydrogenases. Two forms of the active sites are known. The intriguing dinuclear structure of [NiFe] hydrogenases has been a challenging target for inorganic/organometallic chemists, and several thiolate-bridged dinuclear Ni-Fe complexes have been prepared.

The reaction of 4-substituted cyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate with tetrabutyl-ammonium acetate gives both the ipso and cine acetate-substitution products in aprotic solvents. The isomeric 5-substituted iodonium salt also gives the same mixture of the isomeric acetate products. The reaction is best explained by an elimination-addition mechanism with 4-substituted cyclohexyne as a common intermediate. The cyclohexyne formation was confirmed by deuterium labeling and trapping to lead to [4 + 2] cycloadducts and a platinum-cyclohexyne complex. Cyclohexyne can also be generated in the presence of some other mild bases such as fluoride ion, alkoxides, and amines, though amines are less effective bases for the elimination. Kinetic deuterium isotope effects show that the anionic bases induce the E2 elimination (kH/kD > 2), while the amines allow formation of a cyclohexenyl cation in chloroform to lead to E1 as well as SN1 reactions (kH/kD ≈ 1). Bases are much less effective in methanol, and methoxide was the only base to efficiently afford the cyclohexyne intermediate. Nucleophiles react with the cyclohexyne to give regioisomeric products in the ratio dependent on the ring substituent. The observed regioselectivity of nucleophilic addition to substituted cyclohexynes is rationalized from calculated LUMO populations, which are governed by the bond angles at the acetylenic carbons: The less deformed carbon has a higher LUMO population and is preferentially attacked by the nucleophile. [ABSTRACT FROM AUTHOR]

Multiple iron atoms bridged by hydrides is a common structural feature of the active species that have been postulated in the biological and industrial reduction of N 2 . In this study, the reactions of an Fe(II) amide complex with pinacolborane in the presence/absence of phosphines afforded a series of hydride-supported [Fe 4 ] and [Fe 6 ] clusters Fe 4 (μ-H) 4 (μ 3 -H) 2 {N(SiMe 3 ) 2 } 2 (PR 3 ) 4 (PR 3 = PMe 3 (2a), PMe 2 Ph (2b), PEt 3 (2c)), Fe 6 (μ-H) 10 (μ 3 -H) 2 (PMe 3 ) 10 (3), and (η 6 -C 7 H 8 )Fe 4 (μ-H) 2 {μ-N(SiMe 3 ) 2 } 2 {N(SiMe 3 ) 2 } 2 (4), which were characterized crystallographically and spectroscopically. Under ambient conditions, these clusters catalyzed the silylation of N 2 to furnish up to 160 ± 13 equiv of N(SiMe 3 ) 3 per 2c (40 equiv per Fe atom) and 183 ± 18 equiv per 3 (31 equiv per Fe atom). With regard to the generation of the reactive species, dissociation of phosphine and hydride ligands from the [Fe 4 ] and [Fe 6 ] clusters was indicated, based on the results of the mass spectrometric analysis on the [Fe 6 ] cluster, as well as the formation of a diphenylsilane adduct of the [Fe 4 ] cluster.

Solvent-extracted nitrogenase cofactors can reduce C 1 substrates (CN - , CO and CO 2 ) to hydrocarbons in reactions driven by a strong reductant, SmI 2 (E 0' = -1.55 V vs SCE). Here we show that a synthetic [Et 4 N] 4 [Fe 6 S 9 (SEt) 2 ] cluster (designated the Fe 6 RHH -cluster), which mimics the homometallic [Fe 8 S 9 C] core of the nitrogenase cofactor (designated the L-cluster), is capable of conversion of C 1 substrates into hydrocarbons in the same reactions. Comparison of the yields and product profiles between these homometallic clusters and their heterometallic counterparts points to possible roles of the heterometal, interstitial carbide and belt sulfur-bridged iron atoms in catalysis. More importantly, the observation that a "simplified", homometallic cofactor mimic can perform Fischer-Tropsch-like hydrocarbon synthesis suggests future biotechnological adaptability of nitrogenase-based biomimetic compounds for recycling C 1 substrates into useful chemical and fuel products.

Merging cooperative Si-H bond activation and electrophilic aromatic substitution paves the way for C-3-selective indole C-H functionalization under electronic and not conventional steric control. The Si-H bond is heterolytically split by the Ru-S bond of a coordinatively unsaturated cationic ruthenium(II) complex, forming a sulfur-stabilized silicon electrophile. The Wheland intermediate of the subsequent Friedel-Crafts-type process is assumed to be deprotonated by the sulfur atom, no added base required. The overall catalysis proceeds without solvent at low temperature, only liberating dihydrogen.

[NiFe] hydrogenase, the enzyme of which catalyzes the reversible oxidation of molecular hydrogen to protons and electrons, contains a unique heterodinuclear thiolate-bridged Ni-Fe complex in which the iron center is coordinated by CO and CN. We have synthesized dithiolate-bridged Ni-Fe complexes bearing CO and CN ligands to model the active center of [NiFe] hydrogenase. The Ni-Fe complexes containing a [(CN)2(CO)2Fe(mu-S2)NiS2] framework are the closest yet structural models of [NiFe] hydrogenase.