Volume 73, Issue 2

Development of organometallic chemistry in the past half century is reviewed from the author's personal viewpoint with focus on the establishment of fundamental concepts relevant to catalysis. Further development in the coming century is expected in view of the diversity of transition-metal complexes and their unique properties.

The evolution of organometallic chemistry during the second half of the 20 th century has transformed chemical science and technology to a degree and in ways that have rarely been matched throughout the history of chemistry. These include the discovery of radically new types of chemical compounds; novel structures and bonding modes; unprecedented reactivity patterns; unsuspected roles of organometallic chemistry in biology; powerful new synthetic methodologies; new materials; and whole new classes of catalysts and catalytic processes of extraordinary versatility and selectivity. The impact of these developments, which still are unfolding, has been truly revolutionary. Some milestones in this remarkable chapter of chemical history, as well as challenges and opportunities confronting organometallic chemistry today, will be examined.

The factors that control the oxidation of metals in bimetallic complexes are investigated in relation to the possibility of developing one-site addition two-metal oxidation reactions such as those that occur in the respiratory protein,hemerythrin. It is shown that the behavior of bimetallic complexes is not represented by the sum of the analogous monometallic parts because of metal and ligand interactions.

RuCl 2 (phosphine) 2 (1,2-diamine) complexes, coupled with an alkaline base in 2-propanol, allows for preferential hydrogenation of a C=O function over coexisting conjugated or nonconjugated C=C linkages, a nitro group, halogen atoms, and various heterocycles. The functional group selectivity is based on the novel metal-ligand bifunctional mechanism. The use of appropriate chiral diphosphines and diamines results in rapid and productive asymmetric hydrogenation of a range of aromatic, hetero-aromatic, and olefinic ketones. The versatility of this method is manifested by the asymmetric synthesis of various biologically significant chiral compounds.

There has been a recent renaissance in the organometallic chemistry of pentalene, following the discovery of the first complexes incorporating pentalene η 8 -ligated to a single metal center. This short review outlines recent work in the author's laboratory on the preparation of silylated pentalene ligands and the subsequent synthesis of novel, monometallic, and bimetallic pentalene sandwich and half-sandwich complexes of the f- and d-block elements.

The reaction of terminal alkenes with trialkylalanes in the presence of chiral (NMI) 2 ZrCl 2 has been shown to be 70-75% ee for methylalumination and 90-95% ee for ethyl- and higher alkylalumination. It must involve an acyclic bimetallic process. Its scope, limitations, and applications to some natural products syntheses are discussed.

Using organometallic catalysts, two types of polymers containing conjugated moiety and insulating linker are synthesized. The investigations on the photophysical properties of these polymers (photoluminescence, electroluminescence and nonlinear optical properties) are briefly summarized. These polymers represent a new class of materials for optoelectronic interests.

In the palladium (II) -catalyzed enyne coupling reactions, the nitrogen-containing ligand plays the same role as the halide ion to inhibit the β-hydride elimination. Employing the pymox or bisoxazoline as ligands, the catalytic asymmetric cyclization of (Z) -4'-acetoxy-2'-butenyl 2-alkynoates initiated by acetoxypalladation was established with high efficiency (up to 92% ee) to afford the optically active γ-butyrolactones.

The coordination of the functionalities that may be present either in the monomer or in the growing polymer chain to the metal center is a key impediment in the development of new transition metal-based catalysts for the insertion polymerization of polar monomers. Several examples are discussed to illustrate this problem. Additionally, it is shown that the reduction of the electrophilicity of the metal center is one way to decrease this interaction.

"Asymmetric activation" provides a general strategy for the use of racemic catalysts not only with atropos but also tropos ligands without enantiomer resolution.

Dinitrogen complex cis-[W (N 2 ) 2 (PMe 2 Ph) 4 ] reacts with an excess of acidic dihydrogen complexes such as trans-[RuCl (h 2 -H 2 ) (dppe) 2 ]BF 4 (dppe = 1,2-bis (diphenylphosphino) ethane) at 55 °C under 1 atm of H 2 to form ammonia in moderate yield. The reaction is presumed to proceed through nucleophilic attack of the remote nitrogen of the coordinated dinitrogen on the dihydrogen ligand. The coordinated dinitrogen is also protonated by treatment with hydrosulfido-bridged dinuclear complexes such as [Cp*Ir (m-SH) 3 IrCp*]Cl (Cp* = h 5 -C 5 Me 5 ) to afford ammonia. On the other hand, the synthetic cycle for the formation of pyrrole and N-aminopyrrole from dinitrogen and 2,5-dimethoxytetrahydrofuran has been established starting from dinitrogen complexes of the type trans-[M (N 2 ) 2 (dppe) 2 ] (M = Mo, W).

This short review article covers some useful applications of alkynyltungsten compounds to the syntheses of complex lactones. Two types of cyclizations will be emphasized: (1) cycloalkenylation of tungsten-alkynol compounds with aldehydes to give a-alkylidene oxacarbeniums, further leading to a-alkylidene lactones and (2) intramolecular [3+2]-cycloaddition of epoxides to give bicyclic lactones. The new methodologies can provide a short synthesis of enantiopure lactones such as (-) -epilitsenolide C 2 , (+) -listenolide C 1 , (+) -isodihydromahubanolide A, (+) -blastmycinone, and (-) -epi-blastmycinone.

Selective preparation of benzene derivatives from three different alkynes and pyridine derivatives from different alkynes and a nitrile were developed.

Synthesis of molecules combining both fullerene and metallocene moieties separated by some spacer is discussed in connection with the possible intramolecular charge transfer. Two new kinds of spacers, =CH-CO- and -Pd-P-, have been introduced by using the corresponding reactions. Reaction of diazoacetyl ferrocene and cymantrene with C 60 has been studied in detail and products characterized. 1,1'-Bis (diazoacetyl) ferrocene gave with C 60 1:1 adduct to which the ansa-structure has been assigned on the basis of 1 H and 13 C NMR spectra. Palladium-h 2 -C 60 complex with a bidentate ferrocenyl diphosphine as a ligand has been also prepared.

1,3-dialkyl-4,5-dimethylimidazol-2-ylidenes form adducts with various bis (alkylcyclopentadienyl) complexes of magnesium, calcium, strontium, barium, samarium, and ytterbium. The X-ray structures of selected metallocenes and carbene adducts are discussed. Liquid-crystalline homo- and block copolymers with a narrow molecular mass distribution index were obtained in high yields by living polymerization of mesogenic methacrylates with organosamarium complexes. Depending on the structure of the catalyst and the polarity of the solvent used, different, partially competitive reaction mechanisms are observed.

Diels-Alder cycloadditions of S-indoline chiral acrylamides with cyclopentadiene in the presence of Lewis acids proceed with high diastereofacial selectivity, giving either endo-R or endo-S products depending on Lewis acid and the structures of chiral dienophiles. Diastereo- and enantioselective pinacol coupling reactions of chiral α-ketoamides mediated by samarium diiodide afforded extremely high diastereoselectivities. Enantiopure (S,S) - or (R,R) -2,3-dialkyltartaric acid and derivatives can be synthesized. Furthermore, it was demonstrated that α,β-unsaturated amides coupled with SmI 2 to dimerized products containing two chiral carbons which were first obtained as the adjacent chiral carbons.

Buckminsterfullerene, C 60 , is readily reduced on exposure to solutions of strongly reducing transition-metal carbonylate anions to give the radical anion fulleride C 60 - and the corresponding highly reactive, 17-electron neutral compounds. Three secondary reaction paths have been identified, depending on the nature of the reactants and the reaction conditions. (1) With Na + and PPN + salts of [Mn (CO) 5 ] - , thermal substitution of a CO on the metal radical by the C60-results in formation of the anionic, h 2 -fullerene complex [Mn (C 60 ) (CO) 4 ] - . (2) With salts of [Co (CO) 4 ] - , the thermal reaction results in formation of a novel transition-metal fulleride NaCoC 60 while (3) with Na[CpFe (CO) 2 ] and [CpM (CO) 3 ] - (M = Mo, W) , the 17-electron intermediates couple to form the 18-electron dimers, [CpFe (CO) 2 ] 2 and [CpM (CO) 3 ] 2 . In contrast, photochemical reactions of C 60 with salts of [Mn (CO) 5 ] - , [Co (CO) 4 ] - , and [CpM (CO) 3 ] - result in excellent yields of the complexes [Mn (C 60 ) (CO) 4 ] - , [Co (C 60 ) (CO) 3 ] - and [CpM (C 60 ) (CO) 2 ] - , respectively; analogous complexes of C 70 may be made similarly. The new complexes have been characterized crystallographically, by IR, 13 C NMR, and/or Raman spectroscopy and by electrospray mass spectrometry.

Replacement of one of the two C 5 Me 5 ligands in the samarocene (II) complex (C 5 Me 5 ) 2 Sm (THF) 2 with a monodentate anionic ligand (e. g. , OAr, SAr, or NR 1 R 2 ) gives a unique catalytic system, which can not only polymerize styrene and ethylene, but also copolymerize them into block styrene-ethylene copolymers under the presence of both monomers. On the other hand, addition of an appropriate cocatalyst such as MMAO or AlR 3 /[Ph 3 C][B (C 6 F 5 ) 4 ] to the samarocene complex (C 5 Me 5 ) 2 Sm (THF) 2 or (C 5 Me 5 ) 2 Sm (m-Me) 2 AlMe 2 affords a novel catalytic system for stereospecific 1,4-cis living polymerization of butadiene and copolymerization of butadiene with styrene.

The dinuclear Pd (I) complexes containing substitutionally labile acetonitrile ligands [Pd 2 (CH 3 CN) 2 (L) 2 ] (BF 4 ) 2 [L = CH 3 CN (1) , PPh 3 (2) ] were prepared. The complex 1 was demonstrated to serve as a versatile building block for synthesizing sandwich chain complexes made of conjugated polyenes and linear polypalladium chains. The complex 2 reacted with 2 equiv arylacetylenes to give dinuclear organopalladium complexes arising from formal [2s + 2p + 2p] addition with the Pd-Pd s bond participating as one component. The reaction of 2 with 1,3-butadiene afforded a complex regarded as an intermediate lying halfway on the [2s + 4p] reaction coordinate.

New chiral cationic ruthenium complexes have been used for the industrial synthesis of (+) -dihydrojasmonate. A new class of electron-rich C 2 -symmetric 2,4-disubstituted phosphetanes (CnrPHOS) was developed. Preliminary evaluation of their catalytic properties revealed high efficiency in rhodium and ruthenium-catalyzed asymmetric hydrogenations. A new stereochemical model is presented in which the phosphetane Rh-catalyzed hydrogenation follows an apparent stability-controlled mechanism.

The isolobal displacement methodology for the synthesis of biscyclopentadienyl-bridged transition-metal cluster complexes has been developed. This methodology involves various isolobal displacement reactions between tetrahedral cluster substrates and Mo/W-containing isolobal reagents, leading to a series of linear biscyclopentadienyl-bridged cluster complexes and cyclic biscyclopentadienyl-bridged cluster crown ethers, respectively.

Catalytic oxidation of hydrocarbons can be performed efficiently upon treatment with tert-butylhydroperoxide or peracetic acid in the presence of a low-valent ruthenium catalyst. Furthermore, aerobic oxidation of hydrocarbons can be performed in the presence of acetaldehyde using ruthenium, iron, and copper catalysts. Copper derived from copper chloride/crown ether or copper chloride/crown ether/alkaline metal salts have proved to be efficient catalysts. Further study revealed that specific copper complexes formed from copper salts and acetonitrile are convenient and highly useful catalysts for the aerobic oxidation of unactivated hydrocarbons.

Trinuclear polyhydrido complex of ruthenium effectively activates alkanes to cleave C-H bonds in a selective manner due to cooperative action of the metal centers. The reaction of (Cp´Ru) 3 ( m-H) 3 (m 3 -H) 2 (1) (Cp´ = h 5 -C 5 Me 5 ) with n-alkane at 170 °C leads to the formation of a trinuclear closo-ruthenacyclopentadiene complex as a result of a successive cleavage of six C-H bonds. Introduction of a m 3 -sulfido ligand into the Ru 3 core of the trirutheniumpolyhydrido cluster significantly modifies the regioselectivity of the alkane C-H activation. Heating of a solution of (Cp´Ru) 3 (m 3 -S) (m-H) 3 (4) in alkane exclusively gives a trinuclear m 3 -alkylidyne complex via a selective C-H bond cleavage at the less-hindered terminus of alkane molecule.

Several novel and useful reactions involving transition metal-catalyzed C-H bond activation discovered in our laboratory have been summarized here, which includes olefin arylation, hydroarylation of alkynes, carboxylation of arenes and alkanes, and acetoxylation and aminomethylation of alkanes. Possible mechanisms of these reactions have been suggested.

Different types of chiral "privileged" ligands 1 and 2 in promoting enantioselective addition of allylating agents to aliphatic and aromatic aldehydes are described. Here, a new concept in the asymmetric allylation reaction is presented. Redox [Cr (Salen) ] mediated addition of allyl halides to carbonyl compounds is described, and mechanistic investigations are discussed. These results open access to the fascinating area of the catalytic redox processes mediated by metallo-Salen complexes.

Reactions of silanes with d 0 Ta alkylidene and Group 4 amide complexes were found to involve the initial attack on silicon atoms in the silanes by nucleophilic alkylidene carbon or amide nitrogen atoms. The reaction of (Me 3 SiCH 2 ) 3 Ta (PMe 3 ) [=CHSiMe 3 ] (1) with H 2 SiR'Ph (R' = Me, Ph) gave (Me 3 SiCH 2 ) 3 Ta[=C (SiMe 3 ) SiHR'Ph] (2a-b). Similar reactions of these two silanes and (H2PhSi) 2CH2 with (Me 3 SiCH 2 ) Ta (PMe 3 ) 2 [=CHR] 2 (R = SiMe 3 , 3; CMe 3 , 4) yielded novel metallasilacyclobutadienes and a metalladisilacyclohexadiene, respectively. In comparison, the reactions between M (NMe 2 ) 4 (M = Ti, Zr, Hf) and silanes were found to give aminosilanes and metal amide hydride species which were perhaps involved in the formation of titanium-silicon-nitride (Ti-Si-N) ternary materials.

The Pd (0) -catalyzed coupling reaction of aryl and electron-deficient alkenyl halides with the organozinc reagents formed by the subsequent treatment of 1-phenylalk-1-ynes with n-BuLi and ZnBr 2 with or without a catalytic amount of HgCl 2 was studied. Both the allene-formation- and the alkyne-formation-type coupling reactions were observed, depending on the structures of organic halides and 1-arylalk-1-ynes.

Asymmetric homogeneous catalysis forms one of the main planks of modern organic synthesis. It has developed rapidly and largely through the application of novel ligands, whose design is very much based on insight and intuition. At the same time, a better understanding of catalytic reaction mechanisms can contribute to further progress, since it can identify the intimate relationship between ligand structure and successful applications. The presentation will concentrate on the author's research with complexes of the late transition metals and include the search for superior methodologies in hydroboration, as well as ventures into the chemistry of reactive intermediates. The latter will be exemplified from work with rhodium and palladium catalysts.

Planar-chiral heterocycles serve as effective enantioselective nucleophilic catalysts and as useful ligands for asymmetric metal-catalyzed processes.

The synthesis of rare earth metal half-sandwich hydrido complexes [Ln (h 5 :h 1 -C 5 Me 4 SiMe 2 NCMe 3 ) (THF) (µ-H) ] 2 (Ln = Y, Lu) through s-bond metathesis of the easily accessible alkyl complexes [Ln (h 5 :h 1 -C 5 Me 4 SiMe 2 NCMe 3 ) (CH 2 SiMe 3 ) (THF) ] was developed. The dimeric yttrium hydrido complexes are highly fluxional, and a monomer-dimer equilibrium is present. They were tested as single-site, single-component catalysts for the polymerization of ethylene and styrene, as well as alkyl acrylate and acrylonitrile. The hydrido complexes polymerize ethylene slowly and form isolable mono (insertion) products with styrene. The yttrium n-alkyl complexes [Y (h 5 :h 1 -C 5 Me 4 SiMe 2 NCMe 3 ) (R) (THF) ] [R = (CH 2 ) n CH 3 , n = 3-9], prepared by the mono (insertion) of a-olefins, initiate polymerization in a relatively controlled manner. Thus, styrene was polymerized by the n-alkyl complex to give atactic polystyrenes with somewhat enriched syndiotacticities.

Treatment of [60]fullerene with an organocopper reagent converts one of the pentagons of the fullerene into cyclopentadienyl anion through addition of five organic groups on every peripheral carbon atom surrounding the pentagon. Similar treatment of [70]fullerene afforded indenyl anion through regioselective tri-addition of the organic group. These anionic moieties strongly interact with the remainder of the fullerene p-system, and provide unique opportunity for exploration of organometallic chemistry of a new class of metal cyclopentadienides.

This article provides an overview of our work on the synthesis, molecular structure, and bonding of a novel class of 13-vertex closo-metallacarboranes incorporating the h 7 -carboranyl ligand. This bonding mode can only be observed in the d 0 /f n transition-metal ions with the proper size.

The reduction behavior of the novel zirconocene dichlorides, (h5-C 5 Me 5 ) (h 5 -C 5 Me 4 CH 2 CH 2 ER 2 ) ZrCl 2 (ER 2 = NMe 2 , PMe 2 , PPh 2 ) , (h 5 -C 5 Me 4 CH 2 CH 2 PMe 2 ) 2 ZrCl 2 , and (h 5 -C 5 Me 4 CH 2 CH 2 PPh 2 ) (h 5 -C 5 Me 4 CH 2 CH 2 PMe 2 ) ZrCl 2 , together with the new types of intramolecular activation of inert C-H and C-heteroatom bonds are presented and discussed. A remarkably thermally stable zirconocene alkylhydryde type framework complex (h 5 -C 5 Me 5 ) [h 5 ,s-C 5 Me 4 CH 2 CH 2 N (Me) CH 2 -]ZrH, zirconocene arylhydryde complex (h 5 -C 5 Me 5 ) [h 5 ,s-C 5 Me 4 CH 2 CH 2 P (Ph) -o-C 6 H 4 -]ZrH, tetramethylfulvene hydride-type complexes (h 5 -C 5 Me 5 ) [h 6 :h 1 - (CH 2 C 5 Me 3 ) CH 2 CH 2 PR 2 ]ZrH (R = Me, Ph) and (h 5 -C 5 Me 4 CH 2 CH 2 PPh 2 ) [h 6 :h 1 - (CH 2 C 5 Me 3 ) CH 2 CH 2 PMe 2 ]ZrH are reported. The first unusually thermally stable zirconocene (h 5 ,h 1 -C 5 Me 4 CH 2 CH 2 PMe 2 ) 2 Zr with the Zr (II) center stabilized by intramolecular coordination of two Me 2 P-groups is described.

Enantiomerically pure (S,S) -1,2-bis[(o-alkylphenyl) phenylphosphino]ethanes (o-alkylphenyl = o-methylphenyl, o-ethylphenyl, o-isopropylphenyl, 5',6',7',8'-tetrahydro-naphthyl) , (S,S) -1,2-bis (alkyl-methylphosphino) ethanes (alkyl = t-butyl, 1-adamantyl, 1-methylcyclo-hexyl, 1,1-diethylpropyl, cyclopentyl, cyclohexyl) (abbreviated as BisP*) , and (R,R) -1,1-bis (alkylmethylphosphino) methanes (alkyl = isopropyl, t-butyl, cyclohexyl, phenyl) (abbreviated as MiniPHOS) were prepared from phosphorus trichloride via short routes using phosphine-boranes as intermediates. The crystal structures of cationic rhodium complexes, [Rh ((S,S) -1,2-bis ((phenyl) (5',6',7',8'-tetrahydronaphthyl) phosphino) ethane) (cod)]BF 4 , [Rh ((S,S) -t-Bu-BisP* (nbd))]BF4, and [Rh ((R,R) -t-Bu-MiniPHOS) 2 ]PF 6 , clearly indicate the ideal asymmetric environments. These ligands exhibited an excellent to almost perfect level of enantioselectivity in asymmetric hydrogenations of a-dehydroamino acid derivatives and other catalytic asymmetric reactions.

Muons are short-lived species with an elementary positive or negative charge and a mass 207 times that of the electron. These recommendations concern positive muons, given the short lifetime of negative muons. A positive muon mimics a light hydrogen nucleus, and names are given in analogy to existing names for hydrogen-containing compounds. A particle consisting of a positive muon and an electron (µ+ e -) is named "muonium" and has the symbol Mu. Examples: "muonium chloride," MuCl, is the equivalent of deuterium chloride, 2 HCl or DCl; "muoniomethane", CH 3 Mu, is the product of the muoniation of methane;and NaMu is "sodium muonide."

A system of terms applicable to ordered microporous and mesoporous materials is proposed, and rules for writing a standardized crystal chemical formula for such materials are presented. The recommendations are based both on common usage and on a systematic classification scheme. The nomenclature has been developed to encompass all inorganic materials with ordered, accessible pores with free diameters of less than 50 nm. The crystal chemical formula describes the chemical composition of both the guest species and the host, the structure of the host, the structure of the pore system, and the symmetry of the material. This formula can be simplified or expanded to suit the user's requirements.