Metal Atom Synthesis of Organometallic Compounds

Metal Atom Synthesis of Organometallic Compounds, IS, 53 Metal Carbonyl Cations, 8, 117... 

The chemistry of coordination compounds comprises an area of chemistry that spans the entire spectrum from theoretical work on bonding to the synthesis of organometallic compounds. The essential feature of coordination compounds is that they involve coordinate bonds between Lewis acids and bases. Metal atoms or ions function as the Lewis acids, and the range of Lewis bases (electron pair donors) can include almost any species that has one or more unshared pairs of electrons. Electron pair donors include neutral molecules such as H20, NH3, CO, phosphines, pyridine, N2, 02, H2, and ethyl-enediamine, (H2NCH2CH2NH2). Most anions, such as OH-, Cl-, C2042-, and 11, contain unshared pairs of electrons that can be donated to Lewis acids to form coordinate bonds. The scope of coordination chemistry is indeed very broad and interdisciplinary. 

The first report of the preparation of organometallic compounds by direct reaction between metals atoms and organic ligands came in 1969 (727). Research into synthesis using metal atoms has expanded steadily since then and now seems likely to increase very rapidly as the necessary equipment becomes commercially available.1 The field has been reviewed in part before (55, 67, 78a, 84a, 129), but no comprehensive review of the organometallic aspects has yet been published. 

Metal carbonyls belong to a unique class of organometallic compounds where carbon monoxide is bonded to the metal atom through the carbon end. They enjoy their relevance in the synthesis of various complex compounds and cluster compoxmds as well as a potential agent in organic transformations and occasionally catalyze some of the unique chemical transformations. 

Although the development of metal atom vapour technology over the past three decades has shown tremendous utility for the synthesis of a wide range of organometallic compounds (many of which were inaccessible by conventional techniques), the use of this technique for the synthesis of metal alkoxides and related derivatives does not appear to have been fully exploited. In 1990, Lappert et al. demonstrated the utility of this technique for the synthesis of M—O—C bonded compounds by the isolation of alkaline earth metal aryloxides. 

The chemistry of fluorinated 1,3,5-triazines is not as well studied as the chemistry of their chloro derivatives. In case of fluorotriazines the reactions directed on the ring nitrogen atoms, displacement of fluorine atoms and reactions on carbon atoms on the ring with retention of the fluorine atoms appear to be the most characteristic ones. In this section the N-alkylation and N-acylation reactions, as well as replacement of fluorine atoms by a variety of nucleophiles will be considered. Metallation of fluorotriazines and synthesis on the basis of organometallic compounds, as well as the cross-coupling reactions were described. Also several examples of photochemical reactions and transformations are presented. 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

During the catalytic cycle, surface intermediates include both the starting compounds and the surface metal atoms. This working site is a kind of supramolecule that has organometallic character, and, one hopes, the rules of the organometallic chemistry can be valid for this supramolecule. The synthesis of molecular models of these supramolecules makes it possible to study the elementary steps of the heterogeneous catalysis at a molecular level. Besides similarities there are, of course, also differences between the reactivity of a molecular species in solution and an immobilized species. For example, bimo-lecular pathways on surfaces are usually prohibited. 

Aspects of the apparatus for the synthesis using metal atoms are described. The reactions of the atoms of rhenium, tungsten, and osmium with hydrocarbons including alkanes are described. It is shown that metal atom reactions with alkanes can give isolable organometallic compounds including l-alkylidene compounds. 

The report of the first zinc compound with a Zn-Zn core elicited a number of critical comments on the structure and bonding of decamethyldizincocene, and the interpretation of the results.236,237 None of the authors of these commentaries questioned the data or their interpretation. Parkin, however, has pointed out that the formal oxidation state of +1 for zinc in this compound is merely due to the convention that metals are assigned an oxidation state of 0 when they form bonds with like atoms.237 If the conventional definition of valence, namely the capacity of atoms to form bonds to other atoms is used, then the zinc atoms in decamethyldizincocene are not monovalent, but divalent. The synthesis of a paramagnetic organozinc compound in which zinc uses only one of its two 4s electrons will remain an interesting challenge to many synthetic organometallic chemists. 

---