Metal clusters aromaticity

PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

Other Cg hydrocarbons. The dehydrogenation of normal hexane and 2,3-di methyl butane also proceeds but not as voraciously on small platinum clusters. Figure 8 is a plot of the hydrogen content in the first adduct as a function of the size of the platinum metal cluster. The metal atom reacts via dihydrogen elimination to produce PtC6Hi2 products. The platinum trimer is now the smallest cluster that will produce a C H near one. The similarity of size dependent dehydrogenation of the normal hexane and the branched molecule suggest that these systems may not readily aromatize these alkanes. Further structural studies are needed to identify the reaction products. 

Despite its unsaturated nature, benzene with its sweet aroma, isolated by Michael Faraday in 1825 [1], demonstrates low chemical reactivity. This feature gave rise to the entire class of unsaturated organic substances called aromatic compounds. Thus, the aromaticity and low reactivity were connected from the very beginning. The aromaticity and reactivity in organic chemistry is thoroughly reviewed in the book by Matito et al. [2]. The concepts of aromaticity and antiaromaticity have been recendy extended into main group and transition metal clusters [3-10], The current chapter will discuss relationship among aromaticity, stability, and reactivity in clusters. 

Application of this procedure to post-transition metal clusters indicates that bare Ga, In, and Tl vertices contribute one skeletal electron bare Ge, Sn, and Pb vertices contribute two skeletal electrons bare As, Sb, and Bi vertices contribute three skeletal electrons and bare Se and Te vertices contribute four skeletal electrons in two- and three-dimensional aromatic systems (see Chapter 1.1.3). Thus, Ge, Sn, and Pb vertices are isoelectronic with BH vertices and As, Sb, and Bi vertices are isoelectronic with CH vertices. 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

The complex FeCo2(CO)9(PC6H5) is black in the crystalline state and purple in solution. It is moderately air sensitive. It has a low solubility in aliphatic hydrocarbons and a moderately good solubility in aromatic hydrocarbons. In cyclohexane solution it shows Vco bands at 2101 (vs), 2059 (vs), 2048 (vs), 2039(vs), 1981 (w), 1969(w)cm 1. It has been used for various types of basic metal cluster reactions.16... 

Recently, Pt clusters supported on hydrotaleite-derived magnesia, Mg(Al)0, were shown to catalyze the aromatization of n-hexane as effectively as a Pt/KL catalyst (8). A high surface area basic oxide was chosen to support the metal clusters in order to investigate the influence of carrier structure on aromatization. In that study, significant quantities of cracked products were produced by both the zeolite and non-zeolite catalysts. Since the zeolite catalyst was prepared in a fashion that resulted in residual acidity being present, observation of cracked products is not surprising. However, cracking reactions over Pt/Mg(Al)0 are not as easily explained. 

Although the reactions between alkynes or alkenes and metal clusters are the main source of alkyne-substituted complexes, there are other reagents which can produce similar products. Two such reagents are tetraphenylcyclopentadienone, which in the reaction with Ru3(CO)i2 produces Ru3(CO)10(PhCCPh) (167), and dimethyl-vinylarsine, which has been made to react with several carbonyl clusters [Eq. (8)] (168, 169). In the reaction of M3(CO)12 (M = Ru, Os) with a number of tertiary phosphines and aromatic alcohols, an oxidative addition takes place and benzyne-triosmium compounds are obtained (170-176). The fact that Os3(CO)uPEt3 can be converted into an alkyne compound (177) suggests that the conversion goes through substituted intermediates. Carbene derivatives of clusters have also... 

Treatment of 3-borolenes or 2-boraindans with bulky lithium amides yields the dihthiated aromatic borolide dianions, which are applicable as hgand precursors for transition-metal complexes. Many borole complexes including a number of unusual multidecker sandwich complexes and mixed-metal clusters have been described. 4 3,2i2 An unexpected new entry into the synthesis of borole complexes has been recently discovered. Bochmann found that attack of B(C6Fs)3 at a zirconium bound diene leads to a pentafluorophenylborole complex throngh snccessive C-H activation steps. ... 

Electronic structure and reactivity of aromatic metal clusters... 

In this work, we discuss the results of DFT calculations for some all-metal clusters with the general formula MAI/ at a validated level of theory and numerical precision and compute a number of accepted properties to describe aromaticity, such as 17, geometrical parameters, the DI, and the NICS indexes. Hereby, we pursue the evaluation of DFT calculations and reactivity descriptors to explain and assess aromaticity in the anionic all-metal clusters derived from the Al/- unit. We determine the effect of different charges and multiplicities on the geometry of Al/(n = —2, —1,0,1) and calculate the structures of new complexes MA14" where M = (Li+, Na+, K+), (Be+2, Mg+2, Ca+2), (Sc+3, Ti+4), and (B+3, Al+3, Ga+3). In order to compare the DFT reactivity descriptors, we compute other parameters (NICS and DIs) and study periodic trends. 

Evaluate the ability of the DFT calculations and reactivity descriptors to explain and assess aromaticity in the anionic all-metal cluster, Al/-. 

Let us briefly analyze the molecular orbitals (MOs) of Al42. It can be seen that, as expected, these MOs are symmetrically quite different from the analogous traditional aromatic cyclic hydrocarbons (Figure 2). This shows that the concept of aromaticity in the all-metal clusters is certainly an extension of the original concept and is not exactly similar. This has been already discussed in the introductory section, but it is important to pinpoint that although other aromaticity descriptors (hardness, DI, and NICS) may go... 

Several indexes of aromaticity known from unsaturated hydrocarbons were applied to these metal clusters, apparently succeeding in describing its all-metal aromaticity The geometry of the Al42 unit preserves a square structure (bond length equalization), the global hardness is in the same magnitude as in traditional aromatic molecules, and NICS and DI indexes also support the aromatic character of these molecules. 

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