Metal atoms, bare, reactions

Until recently, despite the number of the examples of the formation of cobalt(II)- and iron(II)-tetraphenylporphyrins (MTPP) (107,108) from the direct reaction of the bare metal atoms (Co or Fe) and adsorbed tetraphenylporphyrin molecules (2HTPP), it was only possible to speculate (107-110) about the reaction mechanism, since the reaction is fast at 300 K, and thus no intermediates could be observed. 

One of the most difficult problems for ab initio quantum chemistry is to determine the potential energy function for a chemical reaction on a metal surface. Why is this so First of all, the metal substrate is strongly delocalized. This means that the system cannot be modeled [1] by considering just a small or medium-sized cluster of metal atoms. On the other hand, the band structure techniques that would simplify calculations for a bare metal surface cannot be directly applied because the translational symmetry is broken by the presence of the reactants. As a result one has the difficulty of dealing with extended interactions without the benefit of simplifications due to symmetry. Many problems involving surfaces, interfaces, impurities, or defects in solid state materials fall under this broad rubric along with various solution phenomena as well. 

Oxidative addition reactions of bare metal atoms (Mn, Fe, Co, Ni, Cu, Zn, Ag, Au) with alkanes are observed upon irradiation of the metal in alkane matrices . For example, iron atoms in their excited state insert spontaneously into the C-H bonds of methane ... 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

The study of gas-phase activation of H-H, C-H and C-C bonds of the hydrogen molecule and saturated hydrocarbons, respectively, by bare transition metal atoms and cations is very attractive for getting insight to the mechanisms and factors (such as nature of metal atoms and their lower-lying electronic states) controlHng catalytic activities of transition metal complexes. Such studies, which are free from the ligand and solvent effects, have been subject of many experimental [2] and theoretical [3] papers in the past 10-15 years. Experimental studies indicate that reaction of some transition metal cations (such as Fe+, Co+, and Rh ) with methane exclusively leads to the ion-molecule complex M+(CH4), while others (such as Sc+ and Ir ) pro-... 

A property of superoxide to act as an oxidizing agent is much more ambiguous. Thermodynamically, an electron transfer to bare superoxide is almost impossible because the product of this reaction, peroxide dianion (02 ), is highly imstable. Therefore, the reduction of superoxide is either a proton-coupled process (Eq. (3)) or metal-assisted reaction (Eq. (4)), where the latter requires coordination of 02 and subsequent inner-sphere electron transfer. It is also possible to think in terms of hydrogen atom transfer reactions as a special sort of proton-coupled electron-transfer processes (Eq. (5)). 

Two reviews address the reactions of bare transition metal atoms and ions with hydrocarbons in the gas phase and the reactions of monosubstituted alkanes with bare transition metal ions. 2 A study of the reactivity of ground-state, neutral transition metal atoms from the left hand side of the 4d series (Y through Mo) shows that they are unreactive towards linear alkanes but that they will react with cyclopropane and alkenes.3 Atomic metal cations form a 1 1 adduct with tribenzocyclotriyne in a Fourier-transform ion cyclotron resonance spectrometer. Reaction of molecular oxygen with M(C2H4) results in ligand exchange to M02 for the early first row transition metals. Activation of the O—O bond and product formation is observed for ccnnplexes of Sc+,Ti+andV+.5... 

Metal Atom Chemistry Oxidative addition reactions of bare metal atoms with alkanes have been observed on irradiation in an alkane matrix at low temperatures for a number of transition metal atoms and in metal vapor synthesis in which metal atoms and alkanes are cocondensed at low temperatures. A number of naked metal ions in the gas phase undergo reactions with alkanes in a mass spectrometer chamber the structures of the products are somewhat conjectural because they are deduced from their mass alone. 

In order to understand the activity of a catalyst, one must first have a decent understanding of the reaction mechanism. To this end, smdies have been performed on the activation of a number of different bonds by the most simple model catalyst a bare Pd(0) atom. This enables one to understand the intrinsic reactivity of the metal atom and how the reaction barrier for oxidative addition is influenced by the different bonds to be activated [17,18,40-43]. It was found that the occupation of the metal is of great importance for the oxidative-addition reaction, because charge donation from the occupied metal d orbitals into the substrate s o orbital is... 

Starting from fluoropyrazine, a regioselective synthesis of iodo- and Iributylstannyl substituted fluoropyrazines has been elaborated. Lithiation of fluoropyrazine with stoichiometric amounts of LTMP and iodine afforded the 2-fluoro-3-iodopyrazine 366 (E=I) as sole product otherwise a mixture of mono-, di-, and triiodo derivatives were formed (Scheme 62, Table 14) [147]. In a similar manner, use of tributyltin chloride as electrophile led to mono and di-stannylpyrazines [215]. Formation of compounds 369, 370 and 371 is a result of metalation at the position adjacent to the nitrogen atom without assistance of the fluorine atom as DMG. Such a metalation without a DMG has been previously reported during direct metalation of bare pyrazine by use of an excess of LTMP (4 equiv.) with very short reaction time (5 min) at low temperature -78 °C [216]. 

Bare metallic atoms or metallic atoms covered by hydroxyl ions or water molecules, having a relative coverage (1 - a) constitute the first kind of dissolution site. Two reaction routes have been proposed as being followed on this part of the surface ... 

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